Proposal: add support for Extension frames

h2 currently drops unknown frames . https://www.rfc-editor.org/rfc/rfc9113.html#section-5.5 allows extension frames into HTTP/2, but these cannot be used with h2 due to the unknown frame handling.

There are a few extensions out in the wild as RFCs in various states

• https://www.ietf.org/archive/id/draft-beky-httpbis-metadata-02.html
• https://www.rfc-editor.org/rfc/rfc8336.html
• https://www.rfc-editor.org/rfc/rfc9218.html#section-7.1
• https://www.ietf.org/archive/id/draft-beky-httpbis-metadata-02.html
  and of course its designed to allow arbitrary extensions not just IETF approved ones.

I propose we extend h2 to allow sending and receiving extension frames. I would personally specifically scope this to raw h2 usage, not hyper, as its not clear how these low level concepts could be bubbled up into hyper's API.

Some implementation ideas below...

RecvStream could expose a poll_extension method OR perhaps a poll_data_or_extension if we need to preserve strict ordering of extension and DATA frames. SendStream could similar have a send_extension.

Kind enum Unknown -> Unknown(u8) and Frame enum would get an Extension(..) extra variant. These two changes would be minor breaking changes, but would not break hyper.

From there we can just buffer up the received extension frames to expose to the poll_extension caller, probably with some limits we can come up with. I would probably make it opt-in to enable extension frames so this is no-cost (especially since a user not currently calling poll_extension would never consume the frames).

The send side is likely pretty simple, mostly just some boilerplate to enqueue the new frame type.

Start with src/codec/framed_read.rs and the Kind and Frame enums, then trace the RecvStream and SendStream entry points. Review the HTTP/2 extension-frame rules in RFC 9113 section 5.5 and decide how raw h2 should expose ordering, buffering, limits, and opt-in handling. Done means extension frames can be sent and received through h2 without requiring changes to hyper.