websocket: add WebSocket client module

[hitech95]

Hi,

this adds a new websocket module so ucode scripts can talk to
WebSocket (RFC 6455) servers. Its completly event driven and hooked
into uloop, so it plays nice with the usual event loop based scripts:

import { connect } from 'websocket';
import * as uloop from 'uloop';

uloop.init();

let ws = connect('ws://10.0.0.1:8080/telemetry', {
    headers: { Authorization: 'Bearer ...' },
    max_frame_size: 65536
});

ws.on('open',    (ws) => ws.send('hello'));
ws.on('message', (ws, data, is_text) => print(data));
ws.on('close',   (ws, code, reason) => print(`closed ${code}\n`));
ws.on('error',   (ws, err) => print(`error: ${err}\n`));

uloop.run();

Some notes on the design descisions:

• Framing is done by wslay (MIT), which is a small non-IO library -
  the caller drives the event loop, which fits our embeded use case nicely. It is linked the same way zlib is linked:
  WEBSOCKET_SUPPORT automaticaly defaults on when the libray is found, no vendored code.
• Since ucode arrow functions have no this, the callbacks recieve the connection as explicit first argument.
• Memory was a main concern: there is no per-message allocations in our layer, wslay reuses its fixed 4 KiB buffers and
  recieved message sizes are capped thru max_frame_size (default 256 KiB, oversize messages get closed with 1009).
• Ping/pong and the close handshake (incl. close codes and reason) are handled automaticallly. Close during connect aborts the connection.
• DNS resolving is still synchronus getaddrinfo for now (same as the socket module), everything after that is async.

Tests: there is a cram suite (test_websocket.t) with a small scripted fixture server that speaks the protocol independantly of wslay, covering handshake validation, echo, fragmentation, oversized
frames, resets, floods etc.
Everything was also run thru ASan and cross compiled + runtime tested on aarch64/musl (qemu).

On the OpenWrt side I'm working on a PR adding libwslay to package/libs, plus the ucode-mod-websocket package defininition happy to send those once/if this is acceptted here.

Known limitations: no TLS (wss:// fails with a clear error for now) and no permessage-deflate (on purpose, memory).
Server role is not planned unless someone actualy needs it.

Real world user case why this exist:
Iḿ making a status bridge between CamillaDSP websocket interface and ubus.

Feedback very welcome, especialy on the API surface and the error
handling conventions.

[blogic]

have a look at https://github.com/blogic/uwsc. it does nto depend on an external websocket library and is derived/based ont he websocket server code that @jow- wrote.

[hitech95]

have a look at https://github.com/blogic/uwsc. it does nto depend on an external websocket library and is derived/based ont he websocket server code that @jow- wrote.

I completly missed that repo! I searched for ucode prefix and havent seen that! Have to do some tests on my integration and see if it is indeed a perfect replacement.
I leave this a draft at this point, i'll close it once I did some verification!

[blogic]

@hitech95 if anything is missing let me know we can extend uwsc. I have been using the uwsc code base for several projects over the last months and it is ultra stable for me.