[Woo POS] Coupons: List - Combine local and remote fetching

[iamgabrielma]

We should consider the API of PointOfSaleItemServiceProtocol since we'll want to receive cached items first and then updated ones from the remote.

[dangermattic]

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