[Woo POS] Coupons: Map coupons as POSItems and pass them to POS

[iamgabrielma]

As continuation of #15325 we need to map app coupon models to POSCoupons: POSItems in the PointOfSaleItemService, which will be served into the PointOfSaleItemsController when we need to fetch them in POS mode through the aggregate model

This follows the same design as mapProductsToPOSItems() -> providePointOfSaleItems()

[dangermattic]

🚫 Please add a type label (e.g. type: enhancement) and a feature label (e.g. feature: stats) to this issue.