Add ASGI `pathsend` extension support

[Archmonger]

Currently, ASGI applications must read and send each file chunk in Python to transmit file data. This is inefficient since it steals a lot of GIL time for something that is an IO-bound task.

Pure-python ASGI webservers could implement pathsend as something akin to asyncio.to_thread( os.sendfile, ... ). When wrapping the operating system's syscall in a thread, it would would free up a ton of GIL time, especially for multiple concurrent huge payloads.

If os.sendfile doesn't exist (such as on Windows) then it would need to fall-back to traditional python chunking.

I can PR this behavior, just let me know if you agree.

Ref:

• Re-evaluate "http.response.zerocopysend"  django/asgiref#423
• Add offset and count to Path Send django/asgiref#469
• Added offset and count parameters to pathsend extension django/asgiref#538
• Does uvicorn support pathsend? Kludex/uvicorn#2286 (reply in thread)