Extend configuration.

Source/MQTTnet.Server/Mqtt/MqttServerStorage.cs

@@ -0,0 +1,27 @@
+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using MQTTnet.Server.Configuration;
+
+namespace MQTTnet.Server.Mqtt
+{
+    public class MqttServerStorage : IMqttServerStorage
+    {
+        private readonly SettingsModel _settings;
+
+        public MqttServerStorage(SettingsModel settings)
+        {
+            _settings = settings ?? throw new ArgumentNullException(nameof(settings));
+        }
+
+        public Task SaveRetainedMessagesAsync(IList<MqttApplicationMessage> messages)
+        {
+            return Task.CompletedTask;
+        }
+
+        public Task<IList<MqttApplicationMessage>> LoadRetainedMessagesAsync()
+        {
+            return Task.FromResult<IList<MqttApplicationMessage>>(null);
+        }
+    }
+}