Apply promotion codes and gift vouchers when creating a Bookeo booking

Steps

1. Collect the customer's promo/coupon code text exactly as they enter it, without trying to parse or validate it client-side
2. Pass it in the promotionCodeInput field on the POST /holds or POST /bookings request body (comma-separate multiple coupon codes if the promotion type allows it)
3. Pass any gift voucher code in the giftVoucherCodeInput field; only one specific (service-tied) voucher can be applied per booking, but multiple generic value-based codes can be comma-separated
4. Read back promotionApplicable and appliedPromotionDiscount from the hold response to confirm a discount was actually applied before showing a price to the customer
5. Read applicableGiftVoucherCredit on the hold response to see the credit that will be deducted if the booking is saved
6. After booking creation, read promotionName, couponCodes, and specificVoucherCode on the booking object to display what was redeemed