fix(auth): add CMS support to billing-and-subscriptions

Because:

- The billing-and-subscriptions API needs to return pricing information
  for the currency used for subscriptions using multi-currency prices

This commit:

- Based on the IAP store ID, it uses the IAP CMS config to determine the
  correct Stripe price to fetch and return priceInfo for the currency
  used for that IAP subscription.
- If currency could not be determined, return null
- If no amount can be determined for the provided currency, retrun null

Closes #PAY-3237

Author: StaberindeZA