gh-64192: Add *buffersize* to `imap()`/`imap_unordered()` in `multiprocessing.pool...

gh-64192: Add *buffersize* to `imap()`/`imap_unordered()` in `multiprocessing.pool` (GH-136871)

The new argument allows consuming the input iterator lazily, potentially
saving memory, and allowing long/infinite iterators.

It mirrors the *buffersize* argument to `concurrent.futures.Executor.map`
that was added in 3.14.

Co-authored-by: Oleksandr Baltian <oleksandr.baltian@maklai.com.ua>
Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Sasha Baltian <sasha.baltian@hyperexponential.com>