Support Fetching and Redeeming Win-Back Offers on Custom Paywall

[fire-at-will]

This PR exposes four new functions to allow developers to fetch & redeem win-back offers that a subscriber is eligible for in their custom paywalls. These functions only function on iOS 18.0+ and require StoreKit 2 to be used.

Fetching Eligible Win-Back Offers

 public static async getEligibleWinBackOffersForProduct(
    product: PurchasesStoreProduct,
    callback: (winBackOffers: PurchasesWinBackOffer[]) => void,
    errorCallback: (error: PurchasesError) => void
  )

public static async getEligibleWinBackOffersForPackage(
    aPackage: PurchasesPackage,
    callback: (winBackOffers: PurchasesWinBackOffer[]) => void,
    errorCallback: (error: PurchasesError) => void
  )

Redeeming Win-Back Offers

public static async purchaseProductWithWinBackOffer(
    product: PurchasesStoreProduct,
    winBackOffer: PurchasesWinBackOffer,
    callback: ({productIdentifier, customerInfo,}: { productIdentifier: string; customerInfo: CustomerInfo; }) => void,
    errorCallback: ({error, userCancelled,}: { error: PurchasesError; userCancelled: boolean; }) => void,
  )

public static async getEligibleWinBackOffersForPackage(
    aPackage: PurchasesPackage,
    callback: (winBackOffers: PurchasesWinBackOffer[]) => void,
    errorCallback: (error: PurchasesError) => void
  )

Other Changes

The PurchaseTester app was updated with a new screen to support testing these flows

Testing

All flows were tested manually through the new screen in the PurchaseTester app.