[Server] Add option to handle header set by a proxy

[FirelightFlagboy]

A Parsec server could be configured behind a reverse proxy (like apache or traefik).
But currently the server does not use the headers added by the proxy (Forwarded-* or X-Forwarded-*).

Important

The server should not use those headers by default, the sysadmin should explicitly enable the server to use those headers.

This could cause issue when the server generate link like for bootstrapping an organization where it only use the scheme of the request instead of looking for the forwarded header:

parsec-cloud/server/parsec/asgi/administration.py

Line 179 in 6a56633

use_ssl=request.url.scheme == "https",

We could take inspiration of keycloak has the optional option --proxy-headers {forwarded|xforwarded}