Keeping track of possible Transport API extensions.

[tomchristie]

Let's try to collate any possible future Transport API extensions here.

Current state...

Request extensions:

• timeout - Dict[float]. With httpcore we currently support timeout configurations of "pool", "connect", "read", "write".

Response extensions:

• reason_phrase - bytes. Not used with HTTP/2+.
• http_version - bytes. Not always available, eg. with WSGI transports, mock transports. Client may default to b"HTTP/1.1".

Future areas that might be supported via Transport API extensions...

• Raw stream read/write interface in response to CONNECT/Upgrade. HTTP/2 bi-directional streaming. #1150
• Interface to support trailing headers. Trailing Headers #1149
• HTTP/2 server push support.
• Potentially stuff like socket address information / certificate info.
• Perhaps zero-copy/sendfile support?
• Low-level debug callbacks. Telemetry support #1529 (comment)
• Support for HTTP/2 never-indexed headers. Support for HTTP/2 "never indexed" headers. #1627