Subscriptions APIs now support PayPal Express

We have added a new payment method type, CustomerPaypalBillingAgreement, to the 2021-07 release candidate that allows the ability to store Paypal Billing Agreements in Shopify to be used by SubscriptionContracts.

This change also introduces two new mutations. One to create a PayPal billing agreement for a customer, and one to update existing agreements. For more information on the new mutations, visit our documentation on customerPaymentMethodPaypalBillingAgreementCreate and customerPaymentMethodPaypalBillingAgreementUpdate