Update order item when quantity is incremented

[alcaprar]

Add new endpoint to increment quantity of an order_item. This should:

• check some preconditions:
  • that there is enough current_availability in the product_sale item ✔️
• atomically: 🚧
  • increment the quanity in the order_item ✔️
  • decrement by one the current_availability of the product_sale ✔️
• return the updated order 🚧