server/rest: add ApiAuth trait to make user auth generic

server/rest: add ApiAuth trait to make user auth generic

This allows switching the base user identification/authentication method
in the rest server. Will initially be used for single file restore VMs,
where authentication is based on a ticket file, not the PBS user
backend (PAM/local).

To avoid putting generic types into the RestServer type for this, we
merge the two calls "extract_auth_data" and "check_auth" into a single
one, which can use whatever type it wants internally.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>