async function addDecodedIdTokenToRequest(idToken, req) {
  try {
    const decodedIdToken = await admin.auth().verifyIdToken(idToken);
    req.user = decodedIdToken;
    functions.logger.log('ID Token correctly decoded', decodedIdToken);
  } catch (error) {
    functions.logger.error('Error while verifying Firebase ID token:', error);
  }
}
Returns a Promise with the Decoded ID Token and adds it to req.user.