moko-billing initial release

[Alex009]

KMM library for work with Play Market and AppStore inapps functionality.
Should allow simplified configuration for common cases and allow to use from common code.

What to do from common code:

• get list of products from store
• start payment process
• restore payments (for iOS)
• get current receipt
• get current available provider (Play Market or AppStore) to correct communicate with server

Common usecases:

• select subscription from list and subscribe/unsubscribe
• select product from list and buy forever
• select product from list and buy consumable item (multiple times buy same item - some inapp currency for example)
• restore already purchased items and subscriptions

Package dev.icerock.moko:billing-inapps