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

• 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.

[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.

[Kludex]

This has been going for long - Sorry about it.

The only blocker here is deciding what to do about the changes on the BaseHTTPMiddleware - but I guess that's really the point of this PR. Would it make sense to remove the extension from the scope?

[Kludex]

Is altering the scope["extensions"] in the application side allowed? Would it be bad to remove the http.response.httpsend from the BaseHTTPMiddleware? It's a question. I'm not sure what would be best.

I would like to avoid adding complexity to the BaseHTTPMiddleware.

[Kludex]

nitpick

Suggested change

	async def _handle_simple(self, scope: Scope, send: Send, send_header_only: bool) -> None:
	await send({"type": "http.response.start", "status": self.status_code, "headers": self.raw_headers})
	if send_header_only:
	await send({"type": "http.response.body", "body": b"", "more_body": False})
	elif "http.response.pathsend" in scope["extensions"]:
	await send({"type": "http.response.pathsend", "path": str(self.path)})
	async def _handle_simple(self, send: Send, send_header_only: bool, send_pathsend: bool) -> None:
	await send({"type": "http.response.start", "status": self.status_code, "headers": self.raw_headers})
	if send_header_only:
	await send({"type": "http.response.body", "body": b"", "more_body": False})
	elif send_pathsend:
	await send({"type": "http.response.pathsend", "path": str(self.path)})