feat: feat: add extraAuthParams (extra authorization request parameters) (#139)

Adds optional extraAuthParams map[string]string config.

Extra params are appended to the authorization request but can never
override plugin-managed params (client_id, state, nonce, etc.).

main.go

@@ -202,6 +202,7 @@ func NewWithContext(ctx context.Context, config *Config, next http.Handler, name
 		}(),
 		forceHTTPS:                config.ForceHTTPS,
 		enablePKCE:                config.EnablePKCE,
+		extraAuthParams:           config.ExtraAuthParams,
 		overrideScopes:            config.OverrideScopes,
 		strictAudienceValidation:  config.StrictAudienceValidation,
 		allowOpaqueTokens:         config.AllowOpaqueTokens,