Add ClientId for application messages published by the server.

Source/MQTTnet/Server/IMqttServerOptions.cs

@@ -4,6 +4,8 @@ namespace MQTTnet.Server
 {
     public interface IMqttServerOptions
     {
+        string ClientId { get; set; }
+
         bool EnablePersistentSessions { get; }
 
         int MaxPendingMessagesPerClient { get; }
@@ -19,6 +21,6 @@ namespace MQTTnet.Server
         MqttServerTcpEndpointOptions DefaultEndpointOptions { get; }
         MqttServerTlsTcpEndpointOptions TlsEndpointOptions { get; }
 
-        IMqttServerStorage Storage { get; }
+        IMqttServerStorage Storage { get; }        
     }
 }

Source/MQTTnet/Server/MqttClientSessionsManager.cs

@@ -364,7 +364,13 @@ namespace MQTTnet.Server
                 return null;
             }
 
-            var interceptorContext = new MqttApplicationMessageInterceptorContext(sender?.ClientId, applicationMessage);
+            var senderClientId = sender?.ClientId;
+            if (sender == null)
+            {
+                senderClientId = _options.ClientId;
+            }
+
+            var interceptorContext = new MqttApplicationMessageInterceptorContext(senderClientId, applicationMessage);
             await interceptor.InterceptApplicationMessagePublishAsync(interceptorContext).ConfigureAwait(false);
             return interceptorContext;
         }

Source/MQTTnet/Server/MqttServerOptions.cs

@@ -8,6 +8,8 @@ namespace MQTTnet.Server
 
         public MqttServerTlsTcpEndpointOptions TlsEndpointOptions { get; } = new MqttServerTlsTcpEndpointOptions();
 
+        public string ClientId { get; set; }
+
         public bool EnablePersistentSessions { get; set; }
 
         public int MaxPendingMessagesPerClient { get; set; } = 250;

Source/MQTTnet/Server/MqttServerOptionsBuilder.cs

@@ -147,6 +147,15 @@ namespace MQTTnet.Server
             return this;
         }
 
+        /// <summary>
+        /// Gets or sets the client ID which is used when publishing messages from the server directly.
+        /// </summary>
+        public MqttServerOptionsBuilder WithClientId(string value)
+        {
+            _options.ClientId = value;
+            return this;
+        }
+
         public IMqttServerOptions Build()
         {
             return _options;