title: In app purchase (IOS and Android) for app having its own authentication
body: <p>I would like to implement an in-app purchase in an existing xamarin forms app that already have it's own authentication system. Is that possible? How can I understand if the logged in user is the one that purchased the product as the purchase is done at Google/Apple Account level? Is there a standard way to deal with this situation? I didn't found any doc about that.</p>