Added Messages for changes on Shopping Lists

You can now subscribe to Messages that are triggered when adding or removing a Line Item from a Shopping List.

Changes:

• [API] Added the ShoppingListLineItemAdded and ShoppingListLineItemRemoved Messages.
• [GraphQL API] Added the following types to the GraphQL schema: ShoppingListLineItemAdded, ShoppingListLineItemRemoved.

The following changes were introduced in terms of GraphQL SDL:

type ShoppingListLineItemAdded implements MessagePayload {
  lineItem: ShoppingListLineItem!
  type: String!
}

type ShoppingListLineItemRemoved implements MessagePayload {
  lineItem: ShoppingListLineItem!
  type: String!
}