[Woo POS] Coupons: Split `POSOrderableItem` into smaller pieces. OrderSynchronizer dependency.

[iamgabrielma]

At the moment POSCartItem requires of its inner item model to be of type POSOrderableItem, since we want to add coupons to cart and this has to be a POSCartItem we need to split the current POSOrderableItem protocol into multiple protocols

This is mainly because we're forced to implement toOrderSyncProductInput(quantity: Decimal) -> OrderSyncProductInput to the remote synchronizer, which accepts .product or .variations only as type. It does not handle coupons.

Ref: a25a648

[dangermattic]

Thanks for reporting! 👍