feat(auth): support private_key_jwt and client_secret_basic (#137)

revocation endpoints, joining the existing client_secret_post default.
Both are opt-in via the new clientAuthMethod config field. Closes #135.

private_key_jwt (RFC 7523 §2.2 / OpenID Connect Core §9)
========================================================
Plugin signs a short-lived JWT with a configured private key and presents
it as client_assertion. Use when the IdP enforces short secret TTLs or
requires secretless client auth (Microsoft Entra ID / Azure AD, Okta,
Auth0, Keycloak).

New Config fields:
  clientAuthMethod          (default: client_secret_post)
  clientAssertionPrivateKey (inline PEM)
  clientAssertionKeyPath    (PEM file path; mutually exclusive)
  clientAssertionKeyID      (JWS kid header — required)
  clientAssertionAlg        (default: RS256; RS/PS/ES 256–512 supported)

PEM forms accepted: PKCS#8, PKCS#1, SEC1.
Assertion claims: iss=sub=clientID, aud=tokenURL, iat=now, exp=now+60s,
random 16-byte hex jti per request. ECDSA signatures are raw r||s per
RFC 7515 (not ASN.1).

client_secret_basic (RFC 6749 §2.3.1)
=====================================
Sends credentials in the Authorization: Basic header instead of the
body. Both halves are form-urlencoded individually before base64 — that
encoding step is required by the spec and is NOT what stdlib's
http.Request.SetBasicAuth does, so the plugin uses its own helper. The
form body omits client_id and client_secret on this path.

Wire-up
=======
Both methods are dispatched at the same two call sites:
  helpers.go:exchangeTokens — auth_code + refresh_token grants
  token_manager.go:RevokeTokenWithProvider — RFC 7009 revocation

Existing clientSecret deployments are unaffected — empty
clientAuthMethod maps to the historical client_secret_post behavior, and
clientAssertion remains nil unless the new fields are set.

Yaegi compatibility
===================
All required crypto/rsa, crypto/ecdsa, crypto/x509, encoding/pem and
crypto/sha256/384/512 symbols are exposed by the traefik/yaegi stdlib
symbol tables (RSA SignPKCS1v15 + SignPSS, ECDSA Sign,
ParsePKCS8/1PrivateKey, ParseECPrivateKey).

Tests (16 new)
==============
Algorithm-family coverage:
  TestIssue135_SignerRSAFamily — RS256/384/512 + PS256/384/512
  TestIssue135_SignerECDSAFamily — ES256/384/512, raw r||s shape
  TestIssue135_SignerRejectsAlgKeyMismatch
  TestIssue135_SignerJTIUniqueness — 50 sigs, all jti distinct
  TestIssue135_SignerPEMVariants — PKCS#8, PKCS#1, SEC1

Config validation:
  TestIssue135_ConfigValidation — full Validate() matrix
  TestIssue135_ConfigKeyPathLoadsFile

Wire-up:
  TestIssue135_AuthCodeExchangeUsesAssertion
  TestIssue135_RefreshTokenUsesAssertion
  TestIssue135_BackcompatClientSecretPath
  TestIssue135_RevocationUsesAssertion
  TestIssue135_BuildSignerFromInlineConfig
  TestIssue135_BuildSignerDefaultsToRS256
  TestIssue135_ClientSecretBasicAuth — Authorization header, no body creds
  TestIssue135_ClientSecretBasicURLEncodesReservedChars — :, +, /, @, =, &
  TestIssue135_ClientSecretBasicRevocation — revocation parity

Documentation
=============
  README.md — required-row note + 5 optional rows + dedicated section
  docs/CONFIGURATION.md — new Client Authentication section with three
    method subsections, OpenSSL keygen snippet, RFC links
  docs/index.html — 5 new config-table rows + Private Key JWT
    explainer card
  .traefik.yml + examples/complete-traefik-config.yaml — commented
    opt-in example

Out of scope (deferred)
=======================
mTLS / tls_client_auth (RFC 8705) — separate change; requires per-call
http.Client with tls.Config.Certificates and conflicts with the current
pooled HTTP client architecture.

docs/index.html

@@ -642,7 +642,7 @@ spec:
                                     </tr>
                                     <tr class="border-b border-gray-100 dark:border-gray-800">
                                         <td class="py-2 px-3"><code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">clientSecret</code></td>
-                                        <td class="py-2 px-3">OAuth 2.0 client secret</td>
+                                        <td class="py-2 px-3">OAuth 2.0 client secret. Only required when <code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">clientAuthMethod</code> is unset or <code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">client_secret_post</code>/<code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">client_secret_basic</code>.</td>
                                     </tr>
                                     <tr class="border-b border-gray-100 dark:border-gray-800">
                                         <td class="py-2 px-3"><code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">sessionEncryptionKey</code></td>
@@ -753,15 +753,48 @@ spec:
                                         <td class="py-2 px-3">false</td>
                                         <td class="py-2 px-3">Require RFC 7662 introspection for opaque tokens</td>
                                     </tr>
-                                    <tr>
+                                    <tr class="border-b border-gray-100 dark:border-gray-800">
                                         <td class="py-2 px-3"><code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">disableReplayDetection</code></td>
                                         <td class="py-2 px-3">false</td>
                                         <td class="py-2 px-3">Disable JTI replay detection (for multi-replica without Redis)</td>
                                     </tr>
+                                    <tr class="border-b border-gray-100 dark:border-gray-800">
+                                        <td class="py-2 px-3"><code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">clientAuthMethod</code></td>
+                                        <td class="py-2 px-3">client_secret_post</td>
+                                        <td class="py-2 px-3">Selects how the plugin authenticates to the token endpoint. One of <code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">client_secret_post</code>, <code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">client_secret_basic</code>, <code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">private_key_jwt</code>.</td>
+                                    </tr>
+                                    <tr class="border-b border-gray-100 dark:border-gray-800">
+                                        <td class="py-2 px-3"><code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">clientAssertionPrivateKey</code></td>
+                                        <td class="py-2 px-3">none</td>
+                                        <td class="py-2 px-3">Inline PEM private key used to sign client assertions for <code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">private_key_jwt</code>.</td>
+                                    </tr>
+                                    <tr class="border-b border-gray-100 dark:border-gray-800">
+                                        <td class="py-2 px-3"><code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">clientAssertionKeyPath</code></td>
+                                        <td class="py-2 px-3">none</td>
+                                        <td class="py-2 px-3">Path to a PEM private key file. Alternative to <code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">clientAssertionPrivateKey</code>.</td>
+                                    </tr>
+                                    <tr class="border-b border-gray-100 dark:border-gray-800">
+                                        <td class="py-2 px-3"><code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">clientAssertionKeyID</code></td>
+                                        <td class="py-2 px-3">none</td>
+                                        <td class="py-2 px-3">JWS <code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">kid</code> header value. Required when <code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">clientAuthMethod</code> is <code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">private_key_jwt</code>.</td>
+                                    </tr>
+                                    <tr>
+                                        <td class="py-2 px-3"><code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">clientAssertionAlg</code></td>
+                                        <td class="py-2 px-3">RS256</td>
+                                        <td class="py-2 px-3">Signing algorithm for the client assertion. One of <code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">RS256</code>/<code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">RS384</code>/<code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">RS512</code>, <code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">PS256</code>/<code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">PS384</code>/<code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">PS512</code>, <code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">ES256</code>/<code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">ES384</code>/<code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">ES512</code>.</td>
+                                    </tr>
                                 </tbody>
                             </table>
                         </div>
                     </div>
+                    <div class="glass p-6 rounded-xl">
+                        <h3 class="font-semibold text-gray-900 dark:text-gray-100 mb-3">Private Key JWT (RFC 7523)</h3>
+                        <p class="text-gray-600 dark:text-gray-400 mb-3 text-sm">Use this when your IdP (Entra ID, Okta, Auth0, Keycloak) pressures short-lived secrets, or when policy mandates secretless service-to-service authentication. The plugin signs a 60-second assertion with the configured private key and sends it as <code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">client_assertion</code> instead of <code class="bg-gray-200 dark:bg-gray-700 px-1 rounded">client_secret</code>. Public-key registration on the IdP replaces shared-secret rotation. See <a href="https://www.rfc-editor.org/rfc/rfc7523" target="_blank" rel="noopener" class="text-gray-600 dark:text-gray-300 hover:text-gray-900 dark:hover:text-gray-100 underline">RFC 7523</a> and <a href="https://github.com/lukaszraczylo/traefikoidc/issues/135" target="_blank" rel="noopener" class="text-gray-600 dark:text-gray-300 hover:text-gray-900 dark:hover:text-gray-100 underline">issue #135</a>.</p>
+                        <pre class="bg-gray-900 text-gray-100 p-4 rounded-lg overflow-x-auto text-sm"><code>clientAuthMethod: private_key_jwt
+clientAssertionKeyPath: /etc/traefik/oidc-client.pem
+clientAssertionKeyID: my-client-key-2026
+# clientSecret no longer required</code></pre>
+                    </div>
                     <div class="glass p-6 rounded-xl">
                         <h3 class="font-semibold text-gray-900 dark:text-gray-100 mb-3">Example: Google Workspace with Domain Restriction</h3>