Added Product key to Line Item

Previously, the Line Items in Carts and Orders contained the platform-assigned 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
}