Introduce load_unknown_x509_certificate()
    
    load_unknown_x509_certificate() serves for the cases where we
    can't be sure what the format of its input certificate is. This
    is the case for installers, it should not be used anywhere else.
    
    https://pagure.io/freeipa/issue/4985
    
    Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Martin Basti <mbasti@redhat.com>