Add an event to receive all application messages at server level.

MQTTnet.Core/Client/MqttApplicationMessageReceivedEventArgs.cs

@@ -0,0 +1,14 @@
+using System;
+
+namespace MQTTnet.Core.Client
+{
+    public sealed class MqttApplicationMessageReceivedEventArgs : EventArgs
+    {
+        public MqttApplicationMessageReceivedEventArgs(MqttApplicationMessage applicationMessage)
+        {
+            ApplicationMessage = applicationMessage ?? throw new ArgumentNullException(nameof(applicationMessage));
+        }
+
+        public MqttApplicationMessage ApplicationMessage { get; }
+    }
+}

MQTTnet.Core/Client/MqttClient.cs

@@ -234,13 +234,7 @@ namespace MQTTnet.Core.Client
                 _processedPublishPackets.Add(publishPacket.PacketIdentifier);
             }
 
-            var applicationMessage = new MqttApplicationMessage(
-                publishPacket.Topic,
-                publishPacket.Payload,
-                publishPacket.QualityOfServiceLevel,
-                publishPacket.Retain
-            );
-
+            var applicationMessage = publishPacket.ToApplicationMessage();
             ApplicationMessageReceived?.Invoke(this, new MqttApplicationMessageReceivedEventArgs(applicationMessage));
         }