Adds lastModifiedAt to LineItem

A new field is introduced to LineItems of Carts, Orders and OrderEdits. This field is updated whenever one of setLineItemShippingDetails, addLineItem, removeLineItem, or changeLineItemQuantity changes the LineItem.

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

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

extend type LineItem {
  lastModifiedAt: DateTime
}