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

MQTTnet.Core/Server/MqttApplicationMessageReceivedEventArgs.cs

@@ -0,0 +1,17 @@
+using System;
+
+namespace MQTTnet.Core.Server
+{
+    public sealed class MqttApplicationMessageReceivedEventArgs : EventArgs
+    {
+        public MqttApplicationMessageReceivedEventArgs(string clientId, MqttApplicationMessage applicationMessage)
+        {
+            ClientId = clientId;
+            ApplicationMessage = applicationMessage ?? throw new ArgumentNullException(nameof(applicationMessage));
+        }
+
+        public string ClientId { get; }
+
+        public MqttApplicationMessage ApplicationMessage { get; }
+    }
+}