Group non-subscription products and expose via Creem API

Currently, non-subscription products cannot be grouped or bundled via the Creem API.

Use case:
Offer a one-time (LTD) product in multiple tiers and allow users to upgrade by paying the price difference (e.g. Basic → Full Kit).

Exposing product group relationships via the API would enable dynamic pricing and upgrade flows without hardcoding product mappings in the application.