Feature: Log in with Google in Flutter App

The starting point for people using the @dwyl Native App is Login.
We are using **Sign in with Google** for the Web App and people will expect it for the Native Mobile app.


# Todo

+ [x] Research how to add basic Google Login to a Flutter App
This looks like a good starting point:
+ [ ] https://pub.dev/packages/google_sign_in
+ [ ] https://medium.com/flutter-community/flutter-implementing-google-sign-in-71888bca24ed
+ [ ] Create a Google App in order to get the API Credentials.
see: https://github.com/dwyl/elixir-auth-google/blob/master/create-google-app-guide.md
+ [x] Display the **Sign in with Google** button in the Flutter App
+ [x] Once the person has _successfully_ authenticated with Google,
display their details as shown in the image above ⬆️
+ [x] Dwyl logo: https://avatars1.githubusercontent.com/u/11708465?s=200&v=4
+ [ ] ***Document ALL the Steps*** so that _anyone_ else can follow.
+ [ ] Follow the example in https://github.com/dwyl/elixir-auth-google
and https://github.com/dwyl/elixir-auth-google-demo
+ [ ] Create a new file in _this_ repository called **`google-login-flutter.md`** to capture your notes.

Try and make the **appearance** as ***similar*** to the **Web App** as possible.
(_if you need any help with colors or logos, please leave comments below with your questions_)