Auth Proxy: Allow to set superadmin via a header

[gelicia]

Discussed in #44000

^{Originally posted by UXabre January 13, 2022}
It seems that not many auth methods have the ability to describe a logged on user as being actually a superadmin; I think only ldap and OAuth seem to support it.

I propose to add another Header for auth proxy which can indicate that this user needs to be considered a superadmin. This way, one could utilize auth proxy to access the API, for instance, to create organizations.

A possible alternative, I think, is to support superadmin access tokens (although it's clunky compared to the auth proxy method; which, to me, is vital to have a perfect SSO solution)