(feat) SSO Integration with Forward Auth

[enchant97]

To allow SSO integration using Forward Auth. Most auth providers like Authelia can handle this.

Example Traefik middleware & router:

# !!! untested example !!!
http:
  middlewares:
    authelia:
      forwardAuth:
        address: http://authelia:9091/api/authz/forward-auth
        trustForwardHeader: true
        authResponseHeaders:
          - Remote-User
  routers:
    myapp:
      rule: "Host(`app.example.com`)"
      service: myapp
      middlewares:
        - authelia

This would then be handled in the app by using "Remote-User" header to decide whether a user is authenticated.