gRPC support (HTTP/2 + HTTP/3) #4

Implement gRPC server support on top of the existing HTTP/2 (and HTTP/3) stack.

Scope

• gRPC framing: 5-byte length-prefixed messages, compression flag.
• Trailers (grpc-status, grpc-message) over HTTP/2 and HTTP/3.
• Unary, server-streaming, client-streaming, and bidirectional-streaming RPC patterns.
• Content-types: application/grpc, application/grpc+proto; optional application/grpc-web for browser clients.
• Deadline propagation via grpc-timeout.
• Integration with PHP coroutines so each stream is a coroutine.
• Optional message compression (gzip).
• PHPT + interop tests against grpc-go reference client.

Acceptance

• grpc-go interop test suite passes for the implemented patterns.
• Works over both HTTP/2 (h2c + h2) and HTTP/3.