Standardize Discounts

[dfabulich]

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.

[hyochan]

@dfabulich Working on updates in #63

[hyochan]

@dfabulich The patch is applied in openiap and across all libraries!

Libraries update:

• expo-iap: feat: add standardized DiscountOffer and SubscriptionOffer types hyochan/expo-iap#302
• react-native-iap: feat: sync with openiap v1.3.12 hyochan/react-native-iap#3125
• flutter_inapp_purchase: feat: sync with openiap v1.3.12 hyochan/flutter_inapp_purchase#608
• kmp-iap: feat: sync with openiap v1.3.12 hyochan/kmp-iap#29
• godot-iap: https://github.com/hyochan/godot-iap