A common platform interface for the in_app_purchase plugin.
This interface allows platform-specific implementations of the in_app_purchase plugin, as well as the plugin itself, to ensure they are supporting the same interface.
To implement a new platform-specific implementation of in_app_purchase, extend InAppPurchasePlatform with an implementation that performs the platform-specific behavior, and when you register your plugin, set the default InAppPurchasePlatform by calling InAppPurchasePlatform.setInstance(MyPlatformInAppPurchase()).
To implement functionality that is specific to the platform and is not covered by the InAppPurchasePlatform idiomatic API, extend InAppPurchasePlatformAddition with the platform-specific functionality, and when the plugin is registered, set the addition instance by calling InAppPurchasePlatformAddition.instance = MyPlatformInAppPurchaseAddition().
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.