A common platform interface for the update_available plugin.
This interface allows platform-specific implementations of the update_available plugin, as well as the plugin itself, to ensure they are supporting the same interface.
To implement a new platform-specific implementation of update_available, extend
UpdateAvailablePlatform with an implementation that performs the platform-specific behavior, and when you register your plugin, set the default
UpdateAvailablePlatform by calling
UpdateAvailablePlatform.instance = MyPlatformUpdateAvailable().