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
  ): Promise<[PurchasesWinBackOffer] | undefined>

public static async getEligibleWinBackOffersForPackage(
    aPackage: PurchasesPackage
  ): Promise<[PurchasesWinBackOffer] | undefined>

Redeeming Win-Back Offers

public static async purchaseProductWithWinBackOffer(
    product: PurchasesStoreProduct,
    winBackOffer: PurchasesWinBackOffer
  ): Promise<MakePurchaseResult>

public static async purchasePackageWithWinBackOffer(
    aPackage: PurchasesPackage,
    winBackOffer: PurchasesWinBackOffer
  ): Promise<MakePurchaseResult>

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.

[fire-at-will]

Here, we call the eligibleWinBackOffersForProductIdentifier for the package's product ID. We do this so that we only have to implement the offer caching logic in PHC once instead of twice for both functions

[MarkVillacampa]

I think RCPurchasesErrorCodeDomain should be a symbol reference, not a string.

RCPurchasesErrorCodeDomain is auto-generated in the Objc bridging header in purchases-ios as RevenueCat.ErrorCode from:

https://github.com/RevenueCat/purchases-ios/blob/51dd267dc5e60c59bae90b4413368f9caf030b94/Sources/Error%20Handling/ErrorCode.swift#L20

[MarkVillacampa]

This might be a good candidate to extract as a helper that builds an "unsupported" error with a custom message

[fire-at-will]

Love this! Made these changes here: e2f467a

[MarkVillacampa]

Objc conventions make me think this WithWinBackOffer expect the next parameter to be the winback offer. I think it might be clearer without that part, and it more closely follows the method signature in PHC.

[fire-at-will]

I tried changing the name of this function to purchaseProduct, but then I started seeing namespace collisions with the other purchaseProduct function:

RNPurchases.purchaseProduct was called with 6 arguments but expects 2 arguments. If you haven't changed this method yourself, this usually means that your versions of the native code and JavaScript code are out of sync. Updating both should make this error go away.

[fire-at-will]

Update in case anyone reads this later: we decided not to make this change since Typescript doesn't have named parameters and since this is an internal API

[MarkVillacampa]

Left some minor comments but functionality looks solid 👍