Make claim used for X-Forwarded-User configurable

[thmo]

It would be nice if one could configure the claim that ends up in X-Forwarded-User to be different from the email claim, e.g., use preferred_username.

See also: thomseddon/traefik-forward-auth/issues/266.

[adyanth]

Would also be nice to have other attributes/claims mapped to headers, such as X-Forwarded-Email or X-Forwarded-Groups.

[dleske]

Still interested in this--does anybody have a solution or an alternative?