Split x509.load_certificate() into PEM/DER functions
    
    Splitting the load_certificate() function into two separate helps
    us word the requirements for the input explicitly. It also makes
    our backend similar to the one of python-cryptography so eventually
    we can swap python-cryptography for IPA x509 module.
    
    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>