new checkoutChargeAmount and remainingBalanceChargeAmount fields on Storefront API

As of GraphQL Admin API version 2022-04, checkoutChargeAmount and remainingBalanceChargeAmount fields have been added to sellingPlanAllocation. These fields indicate how much of a product variant's price must be paid upfront, and how much must be paid at a later date. This field is currently restricted behind the extend_selling_plans_api beta flag.