AsyncFramework support sleeping through context

[godexsoft]

Summary

It is useful for certain long running tasks to be able to pause for arbitrary amounts of time.
Different contexts would implement this differently (e.g. CoroContext could use an async timer and yield instead of actually pausing the thread).

Motivation

ETL ng task manager needs a better way to sleep between attempts when there is nothing to do.

Solution

The stopToken passed to client functions by async framework is currently used just as a stop token. Let's upgrade it into a context object that can act as a stop token but also provides a way to pause the task.

Possibly combine with: #1380