[google_sign_in] Add ability to request serverAuthCode on Android

[ambernardino]

Description

Before this it was not possible to request an authCode from GoogleSignIn in order to authenticate with a backend.

Related Issues

flutter/flutter#16613

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[ambernardino]

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[cyanglaz]

It seems to be similar to #879.
@ambernardino Is your PR trying to do the same thing as the other PR?

[collinjackson]

It seems to be similar to #879.
@ambernardino Is your PR trying to do the same thing as the other PR?

It seems they are solving similar problems. This one is Android-only and is using what's in google-services.json whereas #879 is requiring the developer to pass the serverClientID manually.

It would be good to have this functionality on both iOS and Android if possible.

[cyanglaz]

Closing this PR since it doesn't have iOS implementation.
#2116 seems have both implementations.