Add async iterable support to asyncio.staggered.staggered_race

[alex-semenyuk]

Implements the TODO in Lib/asyncio/staggered.py to support async iterables in the coro_fns parameter using aiter() and anext() functions available

Linked PRs

• gh-137128: Support for async iterables in coro_fns #137129

[kumaraditya303]

staggered_race is a private API and I don't think there is any use-case for async iterable in it, the comment seems outdated.