Add or update items in a shopping list

Once a shopping list is created, shoppers can add new item to it or update existing ones.

This endpoint is used to add and update items in a list.

If the specified sku is not in the list, a new item is added to the list with the given quantity. If the specified sku is already in the list, its current quantity is updated based on the given incQuantity.

Note: At least one item must be in the request body.