Add FFI bindings for libcurl's WebSocket interface

[rdw-software]

Bindings coverage:

• curl_ws_meta
• curl_ws_recv
• curl_ws_send

See https://curl.se/libcurl/c/libcurl-ws.html

[rdw-software]

Reminders: Once this has landed, replace/upgrade the crappy WebSocketTestClient.

Also, extend Autobahn testing to the client implementation once that has been implemented (low priority for now).