WebSockets: support passing through sec-websocket headers

[SteffenDE]

Supersedes #5904.

[josevalim]

Ideally connect_info is not specific to any transport. What if we just include all websocket headers alongside x-headers?

[SteffenDE]

@josevalim well, :x_headers is also not transport agnostic. If I build a transport based on telnet, there are no x_headers. I don't think it makes sense to mix x_headers + sec_websocket. If we didn't have x_headers yet, I'd propose a generic :headers key that one would configure with a list of headers / prefixes:

socket "/socket", AppWeb.UserSocket,
  websocket: [
    connect_info: [:uri, headers: [prefix: ["x-", "sec-websocket-"]]]
  ]

But since we already have x_headers 🤷🏻‍♂️

[josevalim]

@SteffenDE x-headers work for websockets too, except in the browser. Another interpretation is to say that x_headers means "extra headers", and that would fit both x- and websocket prefixes. However, if you prefer to go with this PR, I am ok with it. Your call.