Fix support for enhanced authentication (#2132)

* Implement enhanced authentication for client and server.

* Update samples

* Add abstraction layer

* Merge master

* Use dedicated class for public API.

* Update release notes

* Rename methods to fit naming scheme

* Fix unit tests and samples

Source/MQTTnet.Server/Internal/MqttRetainedMessagesManager.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Collections.ObjectModel;
 using MQTTnet.Diagnostics.Logger;
 using MQTTnet.Internal;