Add more function mappings for MQTTnet Server.

Source/MQTTnet/Server/MqttServerClientUnsubscribedTopicHandlerDelegate.cs

@@ -0,0 +1,31 @@
+using System;
+using System.Threading.Tasks;
+
+namespace MQTTnet.Server
+{
+    public class MqttServerClientUnsubscribedTopicHandlerDelegate : IMqttServerClientUnsubscribedTopicHandler
+    {
+        private readonly Func<MqttServerClientUnsubscribedTopicEventArgs, Task> _handler;
+
+        public MqttServerClientUnsubscribedTopicHandlerDelegate(Action<MqttServerClientUnsubscribedTopicEventArgs> handler)
+        {
+            if (handler == null) throw new ArgumentNullException(nameof(handler));
+
+            _handler = eventArgs =>
+            {
+                handler(eventArgs);
+                return Task.FromResult(0);
+            };
+        }
+
+        public MqttServerClientUnsubscribedTopicHandlerDelegate(Func<MqttServerClientUnsubscribedTopicEventArgs, Task> handler)
+        {
+            _handler = handler ?? throw new ArgumentNullException(nameof(handler));
+        }
+
+        public Task HandleClientUnsubscribedTopicAsync(MqttServerClientUnsubscribedTopicEventArgs eventArgs)
+        {
+            return _handler(eventArgs);
+        }
+    }
+}