[Woo POS] Coupons: Create a `POSCoupon` as `POSItem`

[iamgabrielma]

We'll need to create a POSCoupon type that conforms to POSItem in order to take advantage of the current item abstraction.

We might not need all the props in Networking.Coupon, or we might. As inspiration we can take a look into the existing CouponDetailsViewModel to see what's generally used use-facing, if needed.

[dangermattic]

Thanks for reporting! 👍