List all files in GCS bucket.

def gcs_files(prefix_filter=None):
  """List all files in GCS bucket."""
  top_level_xml_str = download_gcs_file("", prefix_filter=prefix_filter)
  xml_root = ElementTree.fromstring(top_level_xml_str)
  filenames = [el[0].text for el in xml_root if el.tag.endswith("Contents")]
  return filenames