issue/1956: Add custom.yaml support for WebSockets

[JJJ]

Fixes #1956.

[JJJ]

For context, I'm running HomeAssistant behind Mail-in-a-Box's reverse proxy via custom.yaml. It uses the WebSockets API (hence wss:// URI's & persistent connections), and nginx needs to know that it is allowed to automatically attempt to "upgrade" HTTP connections to sockets.

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One caveat that I am uncertain about (and hence left alone) is nginx's Host header setting, and how that intersects with MIAB's pass-http-host flag:

• pass-http-host will set Host to $http_host, but...
• nginx suggests using $host

I suspect most devs expect that the values of these 2 variables are the same, but in actuality one value is derived from the other and potentially modified by nginx (meaning, they're usually equal, but that is not a guarantee).

Ultimately, I chose not to modify the existing flag, with the assumption that if this PR works OK for me with HomeAssistant, it is likely to work for others with similar needs.

TY for your consideration @JoshData & contributors 🫶