s3:auth: allow real plaintext authentication

s3:auth: allow real plaintext authentication

In standalone setups we use the PAM stack to verify
the plaintext authentication, so we need to pass it
down...

There are still production systems out there
(legacy audio/video recording systems...)
using this.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=9705

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Wed Jul 17 11:17:54 UTC 2024 on atb-devel-224