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.

examples/complete-traefik-config.yaml

@@ -101,6 +101,16 @@ http:
           providerURL: "https://auth.example.com"
           callbackURL: "/oauth2/callback"
 
+          # ----------------------------------------------------------------
+          # Optional: switch to RFC 7523 private_key_jwt client auth
+          # (Entra ID, Okta, Auth0, Keycloak). Replaces clientSecret with a
+          # signed JWT assertion. See README for details and PEM formats.
+          # ----------------------------------------------------------------
+          # clientAuthMethod: "private_key_jwt"
+          # clientAssertionKeyPath: "/etc/traefik/oidc/client-key.pem"
+          # clientAssertionKeyID: "prod-key-2026"
+          # clientAssertionAlg: "RS256"   # or PS256/384/512, ES256/384/512
+
           # Session Configuration
           sessionEncryptionKey: "prod-encryption-key-64-chars-long-keep-it-secret-and-safe"
           sessionMaxAge: 28800  # 8 hours