Introduced 'key' field on CustomLineItem

We introduced the key field on CustomLineItem and CustomLineItemDraft. This ensures a consistent experience when working with external identifiers across different entity types in Composable Commerce.

Changes:

• [API] Added the key field to CustomLineItem and CustomLineItemDraft.
• [API] Added key field to Add CustomLineItem update action on Carts.
• [API] Added key field to Add CustomLineItem update action on Order Edits.
• [GraphQL API] Changed the CustomLineItemDraftOutput type:
  • Added the key field to the CustomLineItemDraftOutput type.
• [GraphQL API] Changed the AddCartCustomLineItem type:
  • Input field key was added to AddCartCustomLineItem type.
• [GraphQL API] Changed the AddStagedOrderCustomLineItem type:
  • Input field key was added to AddStagedOrderCustomLineItem type.
• [GraphQL API] Changed the CustomLineItem type:
  • Added the key field to the CustomLineItem type.
• [GraphQL API] Changed the CustomLineItemDraft type:
  • Input field key was added to CustomLineItemDraft type.

The following changes were introduced in terms of GraphQL SDL:

extend type CustomLineItemDraftOutput {
  key: String
}

extend type CustomLineItem {
  key: String
}

extend input AddCartCustomLineItem {
  key: String
}

extend input AddStagedOrderCustomLineItem {
  key: String
}

extend input CustomLineItemDraft {
  key: String
}