Added UserName property to various Server EventArgs. (#2123)

* Added UserName property to various Server EventArgs.

* InjectedMqttApplicationMessage: Add SenderUserName property.

* Reformat code

* Reformat code

---------

Co-authored-by: christian <6939810+chkr1011@users.noreply.github.com>

Source/MQTTnet.Server/Events/InterceptingSubscriptionEventArgs.cs

@@ -15,12 +15,14 @@ namespace MQTTnet.Server
         public InterceptingSubscriptionEventArgs(
             CancellationToken cancellationToken,
             string clientId,
+            string userName,
             MqttSessionStatus session,
             MqttTopicFilter topicFilter,
             List<MqttUserProperty> userProperties)
         {
             CancellationToken = cancellationToken;
             ClientId = clientId;
+            UserName = userName;
             Session = session;
             TopicFilter = topicFilter;
             UserProperties = userProperties;
@@ -37,6 +39,11 @@ namespace MQTTnet.Server
         /// </summary>
         public string ClientId { get; }
 
+        /// <summary>
+        /// Gets the user name of the client.
+        /// </summary>
+        public string UserName { get; }
+
         /// <summary>
         ///     Gets or sets whether the broker should close the client connection.
         /// </summary>