Support Remote-User Header Auth

[dougmaitelli]

I use a reverse proxy in front of all the dozens of services I host, the services themselves are on a private virtual network and are not reachable from any other means other than the reverse proxy.
Usually I enable Remote-User Header Auth for every service I can so I can use 1 single point of authentication and authorization to manage all my users and accesses.

It would be very nice if Pelican can support that, so when the reverse proxy passes a Remote-User header it automatically authenticate the matching use if it exists.