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>