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

Source/MQTTnet/Client/Connecting/IMqttClientConnectedHandler.cs

@@ -0,0 +1,9 @@
+using System.Threading.Tasks;
+
+namespace MQTTnet.Client.Connecting
+{
+    public interface IMqttClientConnectedHandler
+    {
+        Task HandleConnectedAsync(MqttClientConnectedEventArgs eventArgs);
+    }
+}

Source/MQTTnet/Client/Connecting/MqttClientConnectedHandlerDelegate.cs

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