Added Custom Fields to Stores

You can now define custom fields on Stores.

This feature includes the following changes:

• [API] Added store enumeration value to Customizable Resources
• [API] Added custom field to StoreDraft and Store
• [API] Added the setCustomType update action to Update Store.
• [API] Added the setCustomField update action to Update Store.

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

extend type Store {
  custom: CustomFieldsType
}

extend input StoreUpdateAction {
  setCustomType: SetCustomType
  setCustomFields: SetCustomFields
}