Async Sample: Asynchronous programming with async and await in C#

Added sync, serial-async, and parallel-async examples (#3819)

* Added sync, async, and parallel examples

* Finalize working bits, reorganize, etc.

* Drawing a line in the sand... thinking this is nearly done

* Added sync, serial-async, and parallel-async examples.

* Update to .NET 5

* Undo diagnostic changes, that were unintended.

* Oops, missed these...

* Added VB counterparts.

* Updates to the README