Add Windows Thread Pool I/O scheduler

[lewissbaker]

Add an implementation of an I/O scheduler that makes use of the Windows Thread Pool APIs to schedule work and handle I/O completion events.

The current windows_thread_pool implementation supports scheduling CPU-work and timers but does not yet support any file/socket/pipe I/O.

[mediabuff]

is this same as https://github.com/facebookexperimental/libunifex/blob/master/source/win32/windows_thread_pool.cpp

or how different

[lewissbaker]

is this same as https://github.com/facebookexperimental/libunifex/blob/master/source/win32/windows_thread_pool.cpp

The windows_thread_pool implementation currently only supports the scheduler and time_scheduler interfaces (schedule(), schedule_at(), schedule_after(), now()).

This issue is about extending that implementation with support for async file I/O, async pipe I/O and async socket I/O using the Windows Thread Pool APIs.

[mediabuff]

Could this be easily ported from cppcoro implementation?