Add Discount to Checkout

Adds a discount to an existing checkout. Use this endpoint to apply the following discounts: * Apply a manual discount to a cart. In this case, you can distribute the discount between each line item in the cart based on the item value. * Apply a manual discount against a specific line item. * Apply a manual discount at the item-level with a cart-level discount. Notes: * When you call this API, you clear out all existing discounts applied to line items, including product and order-based discounts. * You cannot apply a manual discount to a specific line item if you have already applied a coupon or automatic promotion to it. Required Fields: * `discounted_amount` at the cart-level or at the item-level To prevent lost updates due to concurrent requests overriding changes made by others, it is recommended to enable optimistic concurrency control by including the `version` field in the request payload. If the provided version does not match the version on the server, a conflict error will be returned, which the client can handle accordingly.