feat: accept custom backends in set_async_backend

[jacopoabramo]

Closes #437

• Add an AsyncBackend protocol and export it from psygnal along with QueueItem
• Accept an AsyncBackend instance in set_async_backend and return that same instance
• Raise RuntimeError when setting a custom backend while a different one is set
• Add close() to TrioBackend, so clear_async_backend closes every backend through close()
• Annotate EventLike.wait as returning object
• Document custom backends in the async guide

One nitpick is that a run() method needs to access callback.dereference() and WeakCallback is private so maybe an helper method could shield the need to call it explicitly.

[codspeed]

Merging this PR will not alter performance

✅ 67 untouched benchmarks

---

_{Comparing jacopoabramo:feat/custom-async-backend (03f5986) with main (48aeff1)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.76%. Comparing base (48aeff1) to head (03f5986).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #439   +/-   ##
=======================================
  Coverage   99.76%   99.76%           
=======================================
  Files          23       23           
  Lines        2533     2548   +15     
=======================================
+ Hits         2527     2542   +15     
  Misses          6        6           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[tlambert03]

thanks @jacopoabramo

I'm in favor of this, couple minor nits about what we expose. curious to get your thoughts

[jacopoabramo]

All reasonable. I'll address them asap

[jacopoabramo]

@tlambert03 done.

I've kept your original suggestion of _AsyncCall but to be perfectly honest since it's used in _weak_callback.py it would make more sense to have it there since _async.py doesn't use it. Up to you really, it's just more for form. I don't think it could cause circular import issues...

[tlambert03]

looks good thanks!