Reserve `websocket.extension` name

Nobody has claimed this yet.

Hello. I write extension plugin manager for [websocket](https://github.com/moteus/lua-websockets-extensions) and permessage-deflate extension. I test it with my websocket implementation and it pass all AutoBahn tests (client and server).
I name it like `websocket.extensions`. In fact this module does not do any IO it just encode and decode already readed data and tell what need to add to handshake.
So I think it should be not difficult to add support extensions to your implementation.
You need just add ability to add headers to handshake to add `Sec-Websocket-Extensions` header.
And encode/decode RSV bits.

Start by tracing the websocket handshake and frame encoding/decoding paths in the repository. The issue describes adding the Sec-Websocket-Extensions handshake header and handling RSV bits for extension encode/decode. Done means an extension manager such as permessage-deflate can integrate without breaking existing websocket behavior; no tests or file paths are named.

Written by the indexing model from the issue text.