Support "data_stream" extension for HTTP/2.

[tomchristie]

We should support a new data_stream extension for HTTP/2.

Rather than .read() and .write() mapping directly onto the TCP stream they should instead wrap onto DataFrames over a single stream ID.

This would allow us to support HTTP/2's "extended CONNECT" and WebSockets-over-HTTP/2.

See: https://www.rfc-editor.org/rfc/rfc8441.html

[tomchristie]

I've retitled this slightly, since I think that we want to add this as a new extension.

We can expose the raw "network_stream" in the same way we do for HTTP/1.1 connections, which would allow developers to get additional information such as the ssl context, connected IPs etc. Or to bust everything up by writing HTTP/2 frames or any other nonsense directly to the TCP connection.