Standardize Discounts #59

https://www.openiap.dev/docs/types/ios lists DiscountOffer and Discount as Apple-specific terms, but Google Play has types that are nearly equivalent. These are ripe for standardization.

https://www.openiap.dev/docs/types/android lists SubscriptionOffer (Google's equivalent to Apple's DiscountOffer), and doesn't mention one-time product discounts, which appear on a completely separate page in https://www.openiap.dev/docs/features/discount

DiscountOffer should be a top-level standardized type, with a SubscriptionDiscountOffer subtype, incorporating Google's SubscirptionOffer and Apple's DiscountOffer. Google's one-time product discounts should be available as a standardized DiscountOffer type.