* Fix thread leak in `FileDownloadDelegate`
* `SwiftFormat`
* Add a shared file IO thread pool per HTTPClient
* User bigger thread pool and initlize lazily during first file write
* thread pool is actually not used in tests
* Update documentation
* fix review comments
* make `fileIOThreadPool` internal
* Add test to verify that we actually share the same thread pool across all delegates for a given HTTPClient
Co-authored-by: Cory Benfield <lukasa@apple.com>