multiple realms fix (#102)

* Allow to use multiple realms

This change is a ressurection of PR #88 which can't be merged due to significant refactor of the codebase.

* Fix the autocleanup routine to handle multiple realms correctly, update tests.

* Metadata rediscovery when provider is unavailable for any reason during the start.

This one prevents the permanent 503 from the plugin when OIDC provider was for some reason unavailable during the start.

types.go

@@ -128,8 +128,10 @@ type TraefikOidc struct {
 	suppressDiagnosticLogs     bool
 	firstRequestReceived       bool
 	metadataRefreshStarted     bool
-	allowPrivateIPAddresses    bool // Allow private IP addresses in URLs (for internal networks)
-	minimalHeaders             bool // Reduce headers to prevent 431 errors
+	lastMetadataRetryTime      time.Time  // Track last metadata retry for failed state recovery
+	metadataRetryMutex         sync.Mutex // Protects lastMetadataRetryTime
+	allowPrivateIPAddresses    bool       // Allow private IP addresses in URLs (for internal networks)
+	minimalHeaders             bool       // Reduce headers to prevent 431 errors
 	securityHeadersApplier     func(http.ResponseWriter, *http.Request)
 	scopeFilter                *ScopeFilter // NEW - for discovery-based scope filtering
 	scopesSupported            []string     // NEW - from provider metadata