Subscriptions Contracts APIs: Introduce SubscriptionContractFetchDeliveryOptions and SubscriptionContractSelectDeliveryMethod mutations

As of the 2024-01 release of the GraphQL Customer API, you can fetch the available delivery options for a subscription contract with subscriptionContractFetchDeliveryOptions. And you can select an option from a delivery options result and update the delivery method on a subscription contract with the subscriptionContractSelectDeliveryMethod mutation.