Add support for ASGI `pathsend` extension

[gi0baro]

Summary

As per title, this adds support for ASGI pathsend extension on FileResponse class for servers implementing it.

This is a re-work of #2435 combined with parts of #2616, as discussed in #2613 with @Kludex.

Checklist

• [x] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• [x] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [x] I've updated the documentation accordingly.

[gi0baro]

@Kludex I just rebased from current master for your convenience

[gi0baro]

@Kludex what's your current take on this? Should I rebase so it can be merged, or should we just forget about it?

[Kludex]

@Kludex what's your current take on this? Should I rebase so it can be merged, or should we just forget about it?

I want this to get in, I want to get back to this soon. I can rebase myself.

Sorry the delay here.

[gi0baro]

@Kludex not sure why the coverage fails after the last rebase..

[Kludex]

@Kludex not sure why the coverage fails after the last rebase..

Because we recently set branch = True on coverage.

[gi0baro]

@Kludex sorry for the long delay.

This is now rebased on latest master, and tests/CI is finally green. Let me know if any additional edits are required for merge.

[Kludex]

Sorry the delay. Gradly you caught me at PyCon IT. 😅