Add new factory overloads.

Source/MQTTnet/Server/IMqttServerFactory.cs

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using MQTTnet.Adapter;
 using MQTTnet.Diagnostics;
 
@@ -6,6 +7,8 @@ namespace MQTTnet.Server
 {
     public interface IMqttServerFactory
     {
+        IList<Func<IMqttFactory, IMqttServerAdapter>> DefaultServerAdapters { get; }
+
         IMqttServer CreateMqttServer();
 
         IMqttServer CreateMqttServer(IMqttNetLogger logger);