[in_app_purchase_storekit] Add AppStore.showManageSubscriptions() for StoreKit2 wrapper #159664

Use case

In my app, I would like to cancel subscriptions without a server. This method is essential for enabling users to cancel subscriptions directly within the app.

According to the documentation:

Consider using the system-provided subscription-management UI. Using StoreKit APIs lets you present a consistent experience that helps people manage or cancel their subscriptions without leaving your app. For developer guidance, see showManageSubscriptions(in:).

For details, see:

• Helping people manage their subscriptions | In-app purchase | Apple Developer Documentation
• showManageSubscriptions(in:) | Apple Developer Documentation

Proposal

Add showManageSubscriptions() (Wrapper for StoreKit2's showManageSubscriptions()) to AppStore class.