Added Product key to Line Item

Previously, the Line Items in Carts and Orders contained the Product ID only, to refer to the related Product. From now on, the Line Items have the user-defined Product key also, in case it exists for the related Product. The new field is present on Carts that were created or updated after 2 December 2021 and on Orders created after this date.

Changes:

• [API] Added productKey field to LineItem.
• [GraphQL API] Changed the LineItem type:
  • Added the productKey field to the LineItem type.

The following changes were introduced in terms of GraphQL SDL:

extend type LineItem {
  productKey: String
}