Add HTTP/2 "data_stream" extension

[tomchristie]

Refs encode/httpx#1150

• Initial implementation.
• Does the stream interface here expose the correct API. (Eg. do we want read(max_bytes=...)?)
• Add test case. (Currently failing coverage)
• Add documentation.

[tomchristie]

I'm going to close this off for now...

If there's motivation from anyone to proceed with HTTP/2 bi-directional streams then we'd want:

• To start with a design discussion.
• A pull request that begins with a test case & docs.
• This implementation is a good reference for a resolution, but we also need to not terminate the outgoing data stream when we send the request. We also probably want to avoid altering the _recieve_response_body method in order to support this niche use-case. We can instead have a bit more implementation detail inside the HTTP2DataStream class.