Add server implementation

MQTTnet.Core/MqttApplicationMessage.cs

@@ -0,0 +1,24 @@
+using System;
+using MQTTnet.Core.Protocol;
+
+namespace MQTTnet.Core
+{
+    public class MqttApplicationMessage
+    {
+        public MqttApplicationMessage(string topic, byte[] payload, MqttQualityOfServiceLevel qualityOfServiceLevel, bool retain)
+        {
+            Topic = topic ?? throw new ArgumentNullException(nameof(topic));
+            Payload = payload ?? throw new ArgumentNullException(nameof(payload));
+            QualityOfServiceLevel = qualityOfServiceLevel;
+            Retain = retain;
+        }
+
+        public string Topic { get; }
+
+        public byte[] Payload { get; }
+
+        public MqttQualityOfServiceLevel QualityOfServiceLevel { get; }
+
+        public bool Retain { get; }
+    }
+}