Utility class supporting dynamic topics using topic templates (#1932)

* Add MqttTopicTemplate

The topic template class is a way to deal with dynamic topics that
is less cumbersome and error-prone than String.Format or
other string fu one might use to handle dynamic topics.

Example:

  MyCity/{street}/{crossing}/lights

'street' and 'crossing' would be parameters.

The class and its extension methods are intended to support
dynamic subscription/publication, message-topic matching and
routing. The template syntax is modelled after the well-known
moustache syntax, which conincidentally is also published
as the 'AsyncAPI channel address expressions'.

* Add MQTTnet.Extensions.TopicTemplate to solution

* Apply code style from team shared resharper settings.
Add missing file headers.

* Update release notes

---------

Co-authored-by: christian <6939810+chkr1011@users.noreply.github.com>

Source/MQTTnet.Extensions.Rpc/IMqttRpcClient.cs

@@ -1,3 +1,7 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// 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;
 using System.Collections.Generic;
 using System.Threading;