gh-128041: Add `terminate_workers` and `kill_workers` methods to ProcessPoolExecutor (GH-130849)

This adds two new methods to `multiprocessing`'s `ProcessPoolExecutor`:
- **`terminate_workers()`**: forcefully terminates worker processes using `Process.terminate()`
- **`kill_workers()`**: forcefully kills worker processes using `Process.kill()`

These methods provide users with a direct way to stop worker processes without `shutdown()` or relying on implementation details, addressing situations where immediate termination is needed.

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Sam Gross @colesbury
Commit-message-mostly-authored-by: Claude Sonnet 3.7 (because why not -greg)