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/InterceptingPublishEventArgs.cs

@@ -10,11 +10,12 @@ namespace MQTTnet.Server
 {
     public sealed class InterceptingPublishEventArgs : EventArgs
     {
-        public InterceptingPublishEventArgs(MqttApplicationMessage applicationMessage, CancellationToken cancellationToken, string clientId, IDictionary sessionItems)
+        public InterceptingPublishEventArgs(MqttApplicationMessage applicationMessage, CancellationToken cancellationToken, string clientId, string userName, IDictionary sessionItems)
         {
             ApplicationMessage = applicationMessage ?? throw new ArgumentNullException(nameof(applicationMessage));
             CancellationToken = cancellationToken;
             ClientId = clientId ?? throw new ArgumentNullException(nameof(clientId));
+            UserName = userName;
             SessionItems = sessionItems ?? throw new ArgumentNullException(nameof(sessionItems));
         }
 
@@ -31,6 +32,11 @@ namespace MQTTnet.Server
         /// </summary>
         public string ClientId { get; }
 
+        /// <summary>
+        /// Gets the user name of the client.
+        /// </summary>
+        public string UserName { get; }
+
         public bool CloseConnection { get; set; }
 
         /// <summary>