Use Iterable instead Iterator on iterate_in_threadpool

[jamesbraza]

Summary

Having AsyncContentStream match typing of SyncContentStream, they should both be Iterator or Iterable, not separate.

Given iterate_in_threadpool is typed with Iterator, and returns an AsyncIterator, I think then AsyncContentStream should be an Iterator as well

From mypy inferred types in test_streaming_response_custom_iterable, it seems AsyncIterable is required. So, this PR standardizes on Iterable/AsyncIterable

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[jamesbraza]

Cc @tomchristie if you got time to take a look at this one, it's another bug I encountered recently

[tomchristie]

@jamesbraza you probably want to check in with other encode maintainers for this project. (I'm focused on httpx, httpcore at the moment.)