certmap: add new library libsss_certmap
    
    With this library it would be possible to map certificates and users not
    only by adding the full certificate to the user's LDAP object but by
    adding e.g. only parts like the issuer and subject name. Additionally
    the library is also able to flexible select/match certificates based on
    values in the certificate.
    
    Details about mapping and matching rules can be found in the included
    man page.
    
    Related to https://pagure.io/SSSD/sssd/issue/3050
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>