Add support for hot reloading of client certificates (#1783)

* Refactor Unit Tests

* Add new certificate provider interface instead of read only certificates list.

* Update ReleaseNotes.md

* Fix Unit Tests

* Fix build

* unit testing hot swappable certificates (#1787)

* Move test classes to correct namespace

* Apply code style

---------

Co-authored-by: Sean Hanna <hannasm@users.noreply.github.com>

Source/MQTTnet.Tests/Extensions/WebSocket4Net_Tests.cs

@@ -23,7 +23,7 @@ namespace MQTTnet.Tests.Extensions
 
             using (var client = factory.CreateMqttClient())
             {
-                var options = new MqttClientOptionsBuilder().WithWebSocketServer("ws://a.b/mqtt").WithTimeout(TimeSpan.FromSeconds(2)).Build();
+                var options = new MqttClientOptionsBuilder().WithWebSocketServer(o => o.WithUri("ws://a.b/mqtt")).WithTimeout(TimeSpan.FromSeconds(2)).Build();
                 await client.ConnectAsync(options).ConfigureAwait(false);
             }
         }