refactor: OnMessageReceived in ein Ereignis umgewandelt für bessere Ereignisbehandlung

- `OnMessageReceived` von einem Delegaten in ein Ereignis umgewandelt, um die Ereignisabonnierung und -behandlung zu verbessern.

DigitalData.Auth.Client/ClientParams.cs

@@ -25,7 +25,7 @@ public class ClientParams
     /// </summary>
     public TimeSpan? RetryDelay { get; set; }
 
-    public ClientEvent OnMessageReceived = delegate { };
+    public event ClientEvent OnMessageReceived = delegate { };
 
     internal void TriggerOnMessageReceived(AuthClient client, string issuer, string audience, string key, ILogger? logger = null)
         => OnMessageReceived(client, issuer, audience, key, logger);