New customer input field for the OrderCreate mutation

As of GraphQL Admin API 2025-01 version, we're deprecating the customer_id field in favor of the new customer field which you can use to either associate an existing customer or upsert a customer record.

This provides some added flexibility when creating an order by saving the caller an extra step in fetching the customer ID before initiating the order creation.

Learn more about orderCreate on Shopify.dev.