Update PLU (Price Look-Up) to Menu Item mapping

Menu items are stored in our platform and can be identified by their ID, which is provided by the restaurant partner. When a restaurant is integrated with our POS APIs, an additional ID is used to identify the same item the customer is ordering on the restaurant partner POS systems. These IDs are sometimes referred as "sku", "external_id", or "plu": this - plu is what we call those identifiers that must find a match on the restaurant's POS system. Our menus APIs can be used to upload an entire menu, including their PLU codes. This API, however, is meant to be the way partners can provide and update that mapping between an item and its PLU code.