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.Tests/API/AuthHubTests.cs

@@ -98,7 +98,7 @@ public class AuthHubTests
         var provider_receiver = Build(opt =>
         {
             opt.Url = _hubUrl;
-            opt.Events.OnMessageReceived = (issuer, audience, key, logger) =>
+            opt.OnMessageReceived += (client, issuer, audience, key, logger) =>
             {
                 rcv_issuer = issuer;
                 rcv_audience = audience;