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.

Checklist

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

[Kludex]

We need tests.

@abersheeran is this the only thing needed on the implementation side?

[abersheeran]

@abersheeran is this the only thing needed on the implementation side?

Yes, its actual logic implementation is all on the server side.

[gi0baro]

@Kludex I added a test for this, but it only test the correctness of the messages as TestClient doesn't support pathsend directly. Let me know if this is ok for you.

[Kludex]

Reference of the previous PR regarding this extension: #1288

[gi0baro]

Reference of the previous PR regarding this extension: #1288

🤔 http.response.zerocopysend (ASGI ref) != http.response.pathsend (ASGI ref)

[Kludex]

OHHHH, this is the one you created. Sorry!

[Kludex]

Thanks @gi0baro 🙏