Add server implementation

MQTTnet.Core/Adapter/IMqttServerAdapter.cs

@@ -0,0 +1,14 @@
+using System;
+using MQTTnet.Core.Server;
+
+namespace MQTTnet.Core.Adapter
+{
+    public interface IMqttServerAdapter
+    {
+        event EventHandler<MqttClientConnectedEventArgs> ClientConnected;
+
+        void Start(MqttServerOptions options);
+
+        void Stop();
+    }
+}