option to trust X-Forwarded-Proto header with websocket connections

[totaam]

Similar to #4436 but more fine-grained: the same http proxy can provide both http and https connections.

X-Forwarded-Proto could be used to trust that the client connection is actually encrypted.

We are assuming here that we want things configured this way.
This usually implies that the connection to the proxy is not a security concern.

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-Proto

[totaam]

This is not as useful as I was hoping it was going to be - because the client is the one that has to trust the connection.
That said, the http headers are now exposed for all websocket connections, so the server could also potentially make use of it.