Added Custom Fields to Shipping Methods

You can now add Custom Fields to Shipping Methods via the Shipping Methods API and the GraphQL API endpoints. This allows you to provide more information about your shipping services, for instance, the estimated delivery window, a carrier code, a pick-up point, or the 'track and trace URL format'.

• [API] Added shipping-method enumeration value to Customizable Resources.
• [API] Added custom field to ShippingMethodDraft and ShippingMethod.
• [API] Added the setCustomType update action to the Update ShippingMethod endpoint.
• [API] Added the setCustomField update action to the Update ShippingMethod endpoint.
• [GraphQL API] Added the custom field to the ShippingMethod type.
• [GraphQL API] Added setCustomType and setCustomFields to ShippingMethodUpdateAction type.

Introduced the following changes to the GraphQL schema (in SDL format):

extend type ShippingMethod {
  custom: CustomFieldsType
}

extend input ShippingMethodUpdateAction {
  setCustomType: SetCustomType
  setCustomFields: SetCustomFields
}