Headers emulation

[FZambia]

This feature allows implementing mechanics described in centrifugal/centrifugo#832 (comment)

WebSocket browser API does not allow setting custom HTTP headers which makes implementing authentication for browser WebSocket connections harder. With Centrifugo JWT authentication it works pretty good, but proxy still requires careful thinking each time.

Centrifugo can help here by providing a feature called headers emulation. Centrifugo users can provide a custom headers map to the browser SDK (centrifuge-js) constructor, these headers are then sent in the first message to Centrifugo, and Centrifugo has an option to translate it to the outgoing proxy request native HTTP headers – abstracting away the specifics of WebSocket protocol in a secure way. This can drastically simplify the integration from the auth perspective since the backend may re-use existing code.

I already have MVP, so maybe (I will still evaluate for some time) it will be possible to do sth like this soon in centrifuge-js:

const centrifuge = new Centrifuge(
    "ws://host/connection/websocket",
    {"headers": {"Authorization": "Bearer XXX"}})

And Centrifugo will deliver Authorization as an HTTP header in connect proxy request, and in can deliver in all other proxy request types too.