Update the chaotic-good wire format with some learnings from the past year, and set up things for the next round of changes we'd like to make:
* Instead of a composite FRAGMENT frame, split out CLIENT_INITIAL_METADATA, CLIENT_END_OF_STREAM, MESSAGE, SERVER_INITIAL_METADATA, SERVER_TRAILING_METADATA as separate frame types - this eliminates a ton of complexity in the transport, and corresponds to how we used the wire format in practice anyway.
* Switch the frame payload for metadata, settings to be protobuf instead of HPACK - this eliminates the ordering requirements on interpreting these frames between streams, which I expect to open up some flexibility with head of line avoidance in the future. It's a heck of a lot easier to read and reason about the code. It's also easier to predict the size of the frame at encode time, which lets us treat metadata and payloads more uniformly in the protocol.
* Add a connection id field to our header, in preparation for allowing multiple data connections
* Allow payloads to be shipped on the control channel ('connection id 0') and use this for sending small messages
Closes#37765
COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37765 from ctiller:tiefling 7b57f72367
PiperOrigin-RevId: 695766541
Craig Tiller