UTIL: libsss_secrets: Add an update function
    
    With the secrets responder, we were bound to what API did Custodia
    provide, because one of the core functions of the secrets responder was
    forwarding to Custodia. Since the library is not bound by any such
    limitation, we can add a function to update a secret.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3685
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>