public protocol PurchaseTracker
Implement this protocol to verify whether a specific purchase has been paid for.
You may implement this against whatever receipt system you use, but typically this is StoreKit.
Flint will call this multiple times for each productID that is required in a
so implementations only need to respond to single product requests.
Return whether or not the specified product ID has been paid for (or should be enabled) by the user. If the status is not yet known, the implementation can return
nilto indicate this indeterminate status.
func isPurchased(_ productID: String) -> Bool?