HTTP/2 client cannot manually abort a request

[Dust1404]

We need the server to be aware of the termination of a request, when the client manually close a request.

However, when using httpx=0.27.2 (with httpx.AsyncClient.stream and HTTP/2), we find that the server will not notice the termination until the TCP connection closes. This can be fixed by sending a RST_STREAM frame when closing a request.