Add support for retained application messages

MQTTnet.Core/Adapter/MqttClientDisconnectedEventArgs.cs

@@ -0,0 +1,17 @@
+using System;
+
+namespace MQTTnet.Core.Adapter
+{
+    public class MqttClientDisconnectedEventArgs : EventArgs
+    {
+        public MqttClientDisconnectedEventArgs(string identifier, IMqttCommunicationAdapter clientAdapter)
+        {
+            Identifier = identifier ?? throw new ArgumentNullException(nameof(identifier));
+            ClientAdapter = clientAdapter ?? throw new ArgumentNullException(nameof(clientAdapter));
+        }
+
+        public string Identifier { get; }
+
+        public IMqttCommunicationAdapter ClientAdapter { get; }
+    }
+}