Expose server session items (#1638)

* Expose server session items

* Add support for custom session items for injected application messages

Source/MQTTnet/Server/InjectedMqttApplicationMessage.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
+using System.Collections;
 
 namespace MQTTnet.Server
 {
@@ -13,8 +14,15 @@ namespace MQTTnet.Server
             ApplicationMessage = applicationMessage ?? throw new ArgumentNullException(nameof(applicationMessage));
         }
 
-        public string SenderClientId { get; set; } = string.Empty;
+        public MqttApplicationMessage ApplicationMessage { get; }
 
-        public MqttApplicationMessage ApplicationMessage { get; set; }
+        /// <summary>
+        ///     Gets or sets the session items which should be used for all event handlers which are involved in message
+        ///     processing.
+        ///     If _null_ is specified the singleton session items from the server are used instead.
+        /// </summary>
+        public IDictionary CustomSessionItems { get; set; }
+
+        public string SenderClientId { get; set; } = string.Empty;
     }
 }