Add server auth code flow about xamarin.googleauth

[rhishikeshj]

Thanks for pointing this out Trevor.
I ll definitely take a look and make sure I improve the API.

[sanjogs]

@trevoriancox To get the ServerAuthCode on iOS, Where do we configure SignInOptions for GoogleSignIn?

[rhishikeshj]

Hi @trevoriancox,
Thank you for raising this issue.
I have updated the Android code for the plugin and it now follows the new Authentication mechanism which Google recommends.

Regarding the ServerAuthCode and IdToken APIs :

According to the Google documents on Sign-in integration here : https://developers.google.com/identity/sign-in/ios/backend-auth and https://developers.google.com/identity/sign-in/android/backend-auth, the recommendation is to use the IdToken for authentication on the backend server.
Can you please let me know why IdToken will not work for your backend server authentication ?

[rhishikeshj]

Fixed here : 9b89b8b

[trevoriancox]

@rhishikeshj You are right, thank you for pointing that out! I do only need id_token for my application. This also avoids asking the user for the unneeded "offline" permission.

[reesesm2000]

Reopening this thread. What I need is to get a serverAuthCode from the OAuth2 flow. This means that not only will a token be returned but also a code that can be sent from the client to the backend server. This allows the server to convert that code into a token/refresh token for itself to use when contacting the Google servers without the user using the app.
See this link for more details.
https://developers.google.com/identity/sign-in/ios/offline-access
Would be great for this to be part of the Xamarin.Auth library.