Add async handler interfaces for connected and disconnected to allow async code to execute on connect

Source/MQTTnet/Client/Disconnecting/MqttClientDisconnectedHandlerDelegate.cs

@@ -0,0 +1,31 @@
+using System;
+using System.Threading.Tasks;
+
+namespace MQTTnet.Client.Disconnecting
+{
+    public class MqttClientDisconnectedHandlerDelegate : IMqttClientDisconnectedHandler
+    {
+        private readonly Func<MqttClientDisconnectedEventArgs, Task> _handler;
+
+        public MqttClientDisconnectedHandlerDelegate(Action<MqttClientDisconnectedEventArgs> handler)
+        {
+            if (handler == null) throw new ArgumentNullException(nameof(handler));
+
+            _handler = context =>
+            {
+                handler(context);
+                return Task.FromResult(0);
+            };
+        }
+
+        public MqttClientDisconnectedHandlerDelegate(Func<MqttClientDisconnectedEventArgs, Task> handler)
+        {
+            _handler = handler ?? throw new ArgumentNullException(nameof(handler));
+        }
+
+        public Task HandleDisconnectedAsync(MqttClientDisconnectedEventArgs eventArgs)
+        {
+            return _handler(eventArgs);
+        }
+    }
+}