certmap: add sss_certmap_display_cert_content()
    
    To make debugging and writing certificate mapping and matching rules
    more easy a new function is added to libsss_certmap to display the
    certificate content as seen by libsss_certmap. Please note that the
    actual output might change in future.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>