Use `Iterable` instead `Iterator` on `iterate_in_threadpool` (#2362)

Use `Iterable` instead `Iterator` on `iterate_in_threadpool` (#2362)

* Fixed AsyncContentStream to be AsyncIterator

* Updating isinstance check too

* Standardizing on Iterable/AsyncIterable

* Moved iterate_in_threadpool to make an iter internally

* Added test of iterate_in_threadpool accepting an Iterable

* Renamed arg to iterator, and fixed type hint in return to be AsyncIterator

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Co-authored-by: Marcelo Trylesinski <marcelotryle@gmail.com>