Added 'shippingCustomFields' and 'shippingKey' fields on Cart and Order

You can now retrieve the shippingCustomFields and shippingKey fields on Carts and Orders with Single ShippingMode.

Changes:

• [API] Added the shippingCustomFields field to Cart and Order.
• [API] Added the shippingKey field to Cart and Order.
• [GraphQL API] Changed the Cart type:
  • Added the shippingKey field to the Cart type.
  • Added the shippingCustomFields field to the Cart type.
• [GraphQL API] Changed the Order type:
  • Added the shippingKey field to the Order type.
  • Added the shippingCustomFields field to the Order type.

The following changes were introduced in terms of GraphQL SDL:

extend type Cart {
  shippingCustomFields: CustomFieldsType
  shippingKey: String
}

extend type Order {
  shippingCustomFields: CustomFieldsType
  shippingKey: String
}