PAM: handled multiple certs in the responder
    
    This patch refactors the handling of the certificate and the attributes
    to address the certificate on the Smartcard (module name, token name and
    key id). Instead of using individual variables the values are put into a
    new struct cert_auth_info. Since the new struct can be used as a list
    the PAM responder can now handle multiple certificates on the Smartcard
    and can send the needed data to pam_sss with multiple SSS_PAM_CERT_INFO
    messages.
    
    Unit tests are added to confirm the expected behavior.
    
    Related to https://pagure.io/SSSD/sssd/issue/3560
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>
    Tested-by: Scott Poore <spoore@redhat.com>