Firebase Firestore
- Login with email, signup, listing user, update user
Reference
- https://github.com/firebase/snippets-ios/blob/6e0f6fb63dbd97f61c62522e09ffa16ce580fb9f/firestore/swift/firestore-smoketest/ViewController.swift
- https://medium.com/firebase-developers/ios-firebase-authentication-sdk-email-and-password-login-6a3bb27e0536