Support Fetching and Redeeming Win-Back Offers on Custom Paywall #1134

[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

  static Future<List<WinBackOffer>> getEligibleWinBackOffersForProduct(
    StoreProduct product,
  ) async

static Future<List<WinBackOffer>> getEligibleWinBackOffersForPackage(
    Package package,
  ) async

Redeeming Win-Back Offers

static Future<CustomerInfo> purchaseProductWithWinBackOffer(
    StoreProduct product,
    WinBackOffer winBackOffer,
  ) async

 static Future<CustomerInfo> purchasePackageWithWinBackOffer(
    Package package,
    WinBackOffer winBackOffer,
  ) async

Other Changes

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

Docs

Code snippets for these functions are added to the public docs in this PR: https://github.com/RevenueCat/docusaurus/pull/526

Testing

All flows were tested manually through the new screen in the PurchaseTester app, and the API tester has been updated with the new functions.

[fire-at-will]

We intentionally use the getEligibleWinBackOffersForProduct function here so that the offer caching logic only needs to be implemented once in the PHC layer

[MarkVillacampa]

Looks good 🚀