awaitPromotionalOfferEither
suspend fun <Error class: unknown class>.awaitPromotionalOfferEither(discount: <Error class: unknown class>, storeProduct: <Error class: unknown class>): Either<<Error class: unknown class>, <Error class: unknown class>>
App Store only. Use this method to fetch a PromotionalOffer to use with awaitPurchase.
Return
An Either.Right containing a PromotionalOffer if successful, an Either.Left containing a PurchasesError in case of a failure.