Add Stl.Async-way of awaiting WaitHandle

[AliveDevil]

I'd like to see some general-purpose ready-to-use implementation of Task WaitOneAsync(this WaitHandle, CancellationToken).
Maybe going as far as returning a ValueTask, though that may be challenging here.

Proposed API:

Task WaitOneAsync(this WaitHandle, CancellationToken = default)
Task WaitOneAsync(this WaitHandle, int Timeout, CancellationToken = default)

[AliveDevil]

If this is a worthwhile addition, I can check to implement this in the not-so distant future.

[alexyakunin]

Hi, sorry, just noticed this - IMO this can wait. As far as I remember, Stl.* doesn't await on WaitHandle-s, so I'd rather postpone this until it's going to be necessary.

[alexyakunin]

@AliveDevil ^