[Server] Add new interceptor before messages are added to a queue.

Frameworks/MQTTnet.NetStandard/Server/MqttClientMessageQueueInterceptorContext.cs

@@ -0,0 +1,20 @@
+namespace MQTTnet.Server
+{
+    public class MqttClientMessageQueueInterceptorContext
+    {
+        public MqttClientMessageQueueInterceptorContext(string senderClientId, string receiverClientId, MqttApplicationMessage applicationMessage)
+        {
+            SenderClientId = senderClientId;
+            ReceiverClientId = receiverClientId;
+            ApplicationMessage = applicationMessage;
+        }
+
+        public string SenderClientId { get; }
+
+        public string ReceiverClientId { get; }
+
+        public MqttApplicationMessage ApplicationMessage { get; set; }
+
+        public bool AcceptEnqueue { get; set; } = true;
+    }
+}