Cleanup public API to not expose internal dependencies

[jpc0]

• Move all folly includes from headers (like websocket_transport.hpp) into the .cpp files
• Move internal headers like conn_state.hpp out of /include, possibly into a src/internal/ or src/transport/impl/ directory
• Keep only actual public API headers in /include, and expose them through a single binaryrpc.hpp entry point
• Remove uWebsockets types from public API