Add StoreKit2 button

[pinkisemils]

To aid with developing the server side changes for StoreKit2, I've added a button that generates a StoreKit2 receipt and sends it to the same endpoint on to our API. The button is only added in debug builds. Since this is a manager is let loose on the codebase they do not understand, please do review these changes diligently. XCode will complain about us not listening to transactions succeeding out-of-band - this is to be expected and we will fix this once we move to StoreKit2 properly - the changes I'm introducing here are not intended to be used in production.

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This change is 

[linear]

IOS-978 Add a button to exercise StoreKit2 in debug builds

[acb-mv]

Reviewed 7 of 7 files at r1, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @pinkisemils)

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ios/MullvadVPN/View controllers/Account/AccountViewController.swift line 305 at r1 (raw file):

        }
        guard case let .received(oldProduct) = productState,
              let accountData = interactor.deviceState.accountData

is the let accountData meant to be duplicated?

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ios/MullvadVPN/Classes/AccessbilityIdentifier.swift line 41 at r1 (raw file):

    case logoutButton
    case purchaseButton
    case storekit2Button

nit: capitalisation; i.e., perhaps this should be storeKit2Button?

[pinkisemils]

Reviewable status: 4 of 7 files reviewed, 2 unresolved discussions (waiting on @acb-mv)

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ios/MullvadVPN/Classes/AccessbilityIdentifier.swift line 41 at r1 (raw file):

Previously, acb-mv wrote…

nit: capitalisation; i.e., perhaps this should be storeKit2Button?

You're correct!

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ios/MullvadVPN/View controllers/Account/AccountViewController.swift line 305 at r1 (raw file):

Previously, acb-mv wrote…

is the let accountData meant to be duplicated?

Done.

[acb-mv]

Reviewed 3 of 3 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved

[rablador]

I get a 400 from the API, "INVALID_INPUT". Is that expected before server side is done?

Reviewed 4 of 7 files at r1, 3 of 3 files at r2, all commit messages.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @pinkisemils)

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ios/MullvadVPN/Classes/AccessbilityIdentifier.swift line 41 at r3 (raw file):

    case logoutButton
    case purchaseButton
    case storeKit2Button

Nit: Unless we add E2E tests we don't need this. It doesn't hurt, but if this feature is being removed/renamed/other it seems likely we might forget to clean up this a11y id as well.

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ios/MullvadVPN/View controllers/Account/AccountViewController.swift line 347 at r3 (raw file):

    private func sendReceiptToAPI(accountNumber: String, receipt: VerificationResult<Transaction>) async {
        do {
            // Accessing unsafe payload data because n

because "n"?

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ios/MullvadVPN/View controllers/Account/PaymentAlertPresenter.swift line 39 at r3 (raw file):

    }

    func showAlertForStoreKitError(

We can use showAlertForError() above instead.