Add support for iOS In-App Purchase promotions

[neilco]

This pull request adds support for the additional SKProductTransactionObserver delegates that support promoting In-App Purchase products in the App Store.

This pull request closes #199.

[hyochan]

@neilco Thank you for the contribution. This is really great support!. I've reviewed some of your lines.

[neilco]

@hyochan Thanks for the comments. This is ready for review again. 👍

[neilco]

@hyochan I’ve spotted an issue related to the React Native app lifecycle—if the app is terminated and a promoted product is clicked in the App Store, the app will open but will never receive the PROMOTED_PRODUCT event. This is because the SKPaymentTransactionObserver delegate is called before React Native has had a chance to add any listeners. I’m testing a solution now.

[neilco]

@hyochan All ready for review 🙈

[kristfal]

Question for this: How would one go about getting the full receipt for a promoted product after the purchase? This is relevant for receipt validation.

[neilco]

@kristfal Purchased promoted products are the same as regular purchases so handle the receipt in exactly the same way.

[kristfal]

Ok, thanks for info. Reason I was asking is because the return value of buyPromotedProduct is not documented. Is it of type <Purchase> (same as buyProduct/buySubscription)?

[neilco]

@kristfal buyPromotedProduct returns a Promise that is resolved with a subset of the purchase data, including the transaction receipt. This is exactly the same as buyProduct and buySubscription.

The only difference is that the product choice happens external to the app—the IAPPromotedProductObserver receives a SKProduct and SKPayment from the App Store and buyPromotedProduct adds that SKPayment to the SKPaymentQueue.