Extend interceptors.

Source/MQTTnet/Server/Internal/MqttClientSession.cs

@@ -22,6 +22,8 @@ namespace MQTTnet.Server.Internal
             ApplicationMessagesQueue = new MqttClientSessionApplicationMessagesQueue(serverOptions);
         }
 
+        public event EventHandler Deleted;
+        
         public string ClientId { get; }
 
         public bool IsCleanSession { get; set; } = true;
@@ -44,5 +46,10 @@ namespace MQTTnet.Server.Internal
             status.PendingApplicationMessagesCount = ApplicationMessagesQueue.Count;
             status.Items = Items;
         }
+
+        public void OnDeleted()
+        {
+            Deleted?.Invoke(this, EventArgs.Empty);
+        }
     }
 }