function decodeJwt_(jwt) {
  var payload = jwt.split('.')[1];
  var blob = Utilities.newBlob(Utilities.base64DecodeWebSafe(payload));
  return JSON.parse(blob.getDataAsString());
}
Decodes and returns the parts of the JWT. The signature is not verified.

@param {string} jwt The JWT to decode.
@return {Object} The decoded payload.