Add server events for subscribed and unsubscribed topics. Add pending messages count for connected clients.

Frameworks/MQTTnet.NetStandard/Server/MqttClientSubscribedTopicEventArgs.cs

@@ -0,0 +1,17 @@
+using System;
+
+namespace MQTTnet.Server
+{
+    public class MqttClientSubscribedTopicEventArgs : EventArgs
+    {
+        public MqttClientSubscribedTopicEventArgs(string clientId, TopicFilter topicFilter)
+        {
+            ClientId = clientId ?? throw new ArgumentNullException(nameof(clientId));
+            TopicFilter = topicFilter ?? throw new ArgumentNullException(nameof(topicFilter));
+        }
+
+        public string ClientId { get; }
+
+        public TopicFilter TopicFilter { get; }
+    }
+}