Is your feature request related to a problem? Please describe.
In my local set up I'm using traefik and forwardAuth -- this means that I have a trusted single sign on that goes from traefik -> oauth2-proxy -> keycloak.

Once that authentication occurs (+ authorization as defined in keycloak) users will be granted access to various services -- including portr. Aat that point, currently they then need to connect portr to their github account for a second authentication.

That's not ideal. I'd prefer if there's a simpler flow given I already manage the authentication and authorization.

Describe the solution you'd like
If configured via some sort of environment variables, I'd like for portr to check that it's currently connected to the trusted proxy & if it is accept a particular header as the authenticated user's email. Then create a user if they don't already exist - and if they do exist, allow them to continue as normal.

Describe alternatives you've considered
I'm currently using a patched dockerfile that implements this behavior - patching particular files so that this feature works, but it's very specific to my use environment.

I did think of turning off the traefik auth entirely for portr - relying solely on github -- but I'd really rather not require a github specific flow.

Additional context

The header is X-Auth-Request-Email for oauth2-proxy & Remote-Email by default for authelia.

I think this should also work for other reverse proxies such as caddy & nginx setups.