files: add support for Smartcard authentication
To support certificate based authentication the files provider must be
able to map a certificate to a user during a BE_REQ_BY_CERT request.
Additionally the authentication request should be handled by the PAM
responder code which is responsible for the local Smartcard
authentication. To be consistent with the other backend an authentication
handler is added to the files provider which unconditionally returns the
offline error code telling the PAM responder to handle the
authentication if it has access to the needed credentials.
Related to https://pagure.io/SSSD/sssd/issue/3500
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>