docs: def _get_client_side_certificate_subject():
      """Return the DN from the client side certificate as a D1 subject if a client side
      cert has been configured.
  
      Else return None.
  
      """
      subject = django.core.cache.cache.get('client_side_certificate_subject')
      if subject is not None:
          return subject
      cert_pem = _get_client_side_certificate_pem()
      if cert_pem is None:
          return None
      subject = _extract_subject_from_pem(cert_pem)
      django.core.cache.cache.set('client_side_certificate_subject', subject)
      return subject