A common platform interface for the google_sign_in plugin.
This interface allows platform-specific implementations of the google_sign_in plugin, as well as the plugin itself, to ensure they are supporting the same interface.
To implement a new platform-specific implementation of google_sign_in, extend GoogleSignInPlatform with an implementation that performs the platform-specific behavior, and when you register your plugin, set the default GoogleSignInPlatform by calling GoogleSignInPlatform.instance = MyPlatformGoogleSignIn().
Strongly prefer non-breaking changes (such as adding a method to the interface) over breaking changes for this package.
See https://flutter.dev/go/platform-interface-breaking-changes for a discussion on why a less-clean interface is preferable to a breaking change.