Make use of PurchasesType inside PurchasesDelegate to allow mocking

[AvdLee]

Right now, delegate methods are defined using the strongly typed Purchases. Therefore, we have to call Purchases.configure in case we want to write tests for methods like:

func purchases(
        _ purchases: Purchases,
        readyForPromotedProduct product: StoreProduct,
        purchase startPurchase: @escaping StartPurchaseBlock
    )

Instead, if the above delegate method would've been defined as:

func purchases(
        _ purchases: PurchasesType,
        readyForPromotedProduct product: StoreProduct,
        purchase startPurchase: @escaping StartPurchaseBlock
    )

We could create a mocked PurchasesType to use in our tests. I can see this as another great improvement similar to the TestProducts release.

[RCGitBot]

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[NachoSoto]

Thanks for the suggestion @AvdLee. It would indeed make it a lot easier.

Unfortunately PurchasesType was added after the initial 4.0 release, and there is no way to make this change now in a backwards compatible way, which would require a major release:

We also can't even provide an @availability annotation with renamed:

We'll leave this issue open for our next upcoming major release though, so we can tackle this at the same time as when we drop support for iOS 11.

[BasThomas]

Just wondering if this is something you're considering tackling for the upcoming 5.0 release? :)