auth: add display auid to KeyServer::encode_secrets

http://tracker.ceph.com/issues/9790 Fixes: #9790

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
auth: unit tests for auid display

Signed-off-by: Loic Dachary <loic-201408@dachary.org>