Remove HTTP2 details from generic types shared by HTTP/1.1

[eddyashton]

Trying to simplify these interfaces to make it easier to add blocking commit, noticed 2 redundancies:

• The HTTPResponder type has stream-specific virtual methods, that are unused. I think these should remain in the HTTP2 inheritance stack, visible only to HTTP2-aware bits of code.
• The per-stream responders were needlessly (afaict) stored in the parent RPCSessions object, even though they're keyed per-sessionID. That seems daft, they can be pushed down the stack into HTTP2ServerSession, simplifying their lifetime and lookup.

[Copilot]

Pull request overview

This PR refactors HTTP/2-specific functionality out of generic HTTP types to simplify the codebase and better separate concerns between HTTP/1.1 and HTTP/2 implementations.

• Removes HTTP/2-specific streaming methods from the generic HTTPResponder interface
• Moves per-stream responder storage from the global RPCSessions class into HTTP2ServerSession, where it belongs
• Eliminates unnecessary cross-session synchronization by localizing responder lifetime to individual HTTP/2 sessions

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/http/responder_lookup.h	Deleted - global responder lookup no longer needed as responders are now session-local
src/http/http_session.h	Removed HTTP/2 stream methods that threw std::logic_error from HTTP/1.1 responder
src/http/http2_types.h	Removed dependency on http_responder.h, defined StreamCloseCB locally
src/http/http2_session.h	Moved responder storage into HTTP2ServerSession, removed override keywords from HTTP/2-specific methods, removed global lookup reference
src/enclave/rpc_sessions.h	Removed ResponderLookup inheritance and updated constructor calls to remove responder lookup parameter
include/ccf/http_responder.h	Removed HTTP/2-specific streaming virtual methods and callback typedef from generic interface