UTIL: Add libsss_secrets
Splits pieces of the secrets responder and makes them into a library.
This is done so that other processes than sssd_secrets can access the
secrets database without going through the whole REST API.
Related:
https://pagure.io/SSSD/sssd/issue/3685
Reviewed-by: Fabiano FidĂȘncio <fidencio@redhat.com>