docs: def signature_cert_chain_url(url):
      """Validate URL specified by SignatureCertChainUrl.
  
      See `validate.request` for additional info.
  
      Args:
          url: str. SignatureCertChainUrl header value sent by request.
  
      Returns:
          bool: True if valid, False otherwise.
      """
      r = urlparse(url)
      if not r.scheme.lower() == 'https':
          warnings.warn('Certificate URL scheme is invalid.')
          return False
      if not r.hostname.lower() == 's3.amazonaws.com':
          warnings.warn('Certificate URL hostname is invalid.')
          return False
      if not os.path.normpath(r.path).startswith('/echo.api/'):
          warnings.warn('Certificate URL path is invalid.')
          return False
      if r.port and not r.port == 443:
          warnings.warn('Certificate URL port is invalid.')
          return False
      return True