Option to also unwind async stacks

[weissi]

With Swift Concurrency there might be many asynchronous tasks waiting for a certain condition to be able to continue. Over time it would be nice to optionally support the asynchronous stacks that aren't currently executing on a thread too.

This isn't possible today with any API, crucially we are missing:

1. Enumeration of asynchronous tasks
2. On Linux, the ability to unwind asynchronous stacks fast

But once these are solved, Swift Profile Recorder should include these too.