This was missed in #1879. Without this, we might have been forwarding some private errors instead of ensuring that we only sent `CodeError`s

This is needed to fix RevenueCat/purchases-hybrid-common#229

[`purchases-hybrid-common`](https://github.com/RevenueCat/purchases-hybrid-common) had a [hacky implementation of `MockPurchases`](https://github.com/RevenueCat/purchases-hybrid-common/blob/4.2.1/ios/PurchasesHybridCommon/PurchasesHybridCommonTests/Mocks/MockPurchases.swift) that subclassed `Purchases`.
Because of that, it had 100+ lines of code to initialize the instance, which was never used because all (most) methods were overridden.

This allows a much better way of doing that, **which also enables any SDK user** to mock `Purchases`.

Another benefit of this is that we can see at a glance what the public API is.
Also, all docstrings are now extracted into the protocol, which makes the implementation file shorter. The docs get inherited by the type itself.

All our API testers pass with no changes, and I've also added the updated `RevenueCat-Swift.h` to verify that no public API changed.

Co-authored-by: Toni Rico <antonio.rico.diez@revenuecat.com>