Updates the quantity of tickets for the given line Item. If the new quantity is greater than the old quantity, new tickets are added. If new quantity is less than old quantity, tickets are removed if and only if they are empty. If Attendee info exists for any tickets, it will not let you update the quantity.

POST https://example.com/api/public/sites/{siteId}/shopping-carts/{cartId}/items/{lineItemId}/update-quantity