React Native Stream Group Chat Tutorial: add auth route

add auth route in server.js

// server/server.js
app.post("/auth", async (req, res) => {
  const user_id = req.body.user_id;
  console.log('user ID: ', user_id);
  if (!user_id) {
    return res.status(400);
  } 

  return res.send({
    token: server_side_client.createToken(user_id)
  });
});