Add HTTP/2 CONTINUATION frame support

[gedaiu]

Implements HEADERS frame splitting into CONTINUATION frames when the HPACK-encoded header block exceeds the peer's MAX_FRAME_SIZE, replacing the previous assert(false) stubs.

Introduces an HTTP2FrameFlag enum to replace magic flag constants, fixes the MAX_FRAME_SIZE settings validation error code from FLOW_CONTROL_ERROR to PROTOCOL_ERROR per RFC 7540 §6.5.2, and adds integration tests covering normal responses, edge cases like 204 No Content and empty bodies, and server resilience against malformed connections.

[gedaiu]

Finally, I managed to make the http2 tests pass on Windows. Unfortunately, curl does not support HTTP/2 on Windows, so I had to disable those tests. Feel free to update the tests if you manage to install a proper curl on the Windows runners.

[s-ludwig]

It's really nice to have this at a basic working and tested state now! I didn't have the time to dig into the issues I encountered after manually integrating the code from the HTTP/2 branch on master - so having CURL work like this is a big milestone.

Did you try whether typical GET requests from Chrome/Firefox are also working?

[gedaiu]

I only tested it with curl. I will do more work on this, and when it is closer to the spec, I will switch my apps to use it and test it properly with browsers.

[s-ludwig]

Sounds good. I've also been wondering whether it would make sense to integrate an external test suite, similar to what I've done for DMDScript. That might help to speed up the process of identifying problematic parts of the code.