Add support for REMOTE_USER header about traefik-forward-auth

[thomseddon]

This seems like a good idea, have you seen a similar header elsewhere? I'm just wondering what the best name for the header would be :)

[timhammond]

https://guacamole.apache.org/doc/gug/header-auth.html
https://answers.splunk.com/answers/46803/sso-works-with-anything-but-remote-user.html

It's kind of an older SSO thing, mostly with Apache. Ideally the header would be something configurable. REMOTE_USER is the default header that Apache used. The main thing here is to pass along the username in a header so the application knows who is logged in. Many applications are configurable. But the default probably should be REMOTE_USER.

[thomseddon]

I've just pushed a fix for this, you can find the user in the X-Forwarded-User header :)