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/SessionDeletedEventArgs.cs

@@ -9,9 +9,10 @@ namespace MQTTnet.Server
 {
     public sealed class SessionDeletedEventArgs : EventArgs
     {
-        public SessionDeletedEventArgs(string id, IDictionary sessionItems)
+        public SessionDeletedEventArgs(string id, string userName, IDictionary sessionItems)
         {
             Id = id ?? throw new ArgumentNullException(nameof(id));
+            UserName = userName;
             SessionItems = sessionItems ?? throw new ArgumentNullException(nameof(sessionItems));
         }
 
@@ -20,6 +21,11 @@ namespace MQTTnet.Server
         /// </summary>
         public string Id { get; }
 
+        /// <summary>
+        /// Gets the user name of the session.
+        /// </summary>
+        public string UserName { get; }
+
         /// <summary>
         ///     Gets or sets a key/value collection that can be used to share data within the scope of this session.
         /// </summary>