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.

token_manager.go

@@ -660,11 +660,33 @@ func (t *TraefikOidc) RevokeTokenWithProvider(token, tokenType string) error {
 	}
 	t.logger.Debugf("Attempting to revoke token (type: %s) with provider at %s", tokenType, revocationURL)
 
+	// Read tokenURL with RLock — used as audience for private_key_jwt (RFC 7523 §3).
+	t.metadataMu.RLock()
+	tokenURL := t.tokenURL
+	t.metadataMu.RUnlock()
+
 	data := url.Values{
 		"token":           {token},
 		"token_type_hint": {tokenType},
-		"client_id":       {t.clientID},
-		"client_secret":   {t.clientSecret},
+	}
+	// client_id is sent in the body for every method except client_secret_basic,
+	// where it is carried in the Authorization header per RFC 6749 §2.3.1.
+	if t.clientAuthMethod != "client_secret_basic" || t.clientAssertion != nil {
+		data.Set("client_id", t.clientID)
+	}
+
+	useBasicAuth := false
+	if t.clientAssertion != nil {
+		assertion, err := t.clientAssertion.Sign(tokenURL, t.clientID)
+		if err != nil {
+			return fmt.Errorf("failed to sign client assertion: %w", err)
+		}
+		data.Set("client_assertion_type", "urn:ietf:params:oauth:client-assertion-type:jwt-bearer")
+		data.Set("client_assertion", assertion)
+	} else if t.clientAuthMethod == "client_secret_basic" {
+		useBasicAuth = true
+	} else {
+		data.Set("client_secret", t.clientSecret)
 	}
 
 	req, err := http.NewRequestWithContext(context.Background(), "POST", revocationURL, strings.NewReader(data.Encode()))
@@ -674,6 +696,9 @@ func (t *TraefikOidc) RevokeTokenWithProvider(token, tokenType string) error {
 
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 	req.Header.Set("Accept", "application/json")
+	if useBasicAuth {
+		setOAuthBasicAuth(req, t.clientID, t.clientSecret)
+	}
 
 	// Send the request with circuit breaker protection if available
 	var resp *http.Response