Return the product's product_quantity field

[raphodn]

Problem

Last october 2024 a new product_quantity field was added in openfoodfacts-server : openfoodfacts/openfoodfacts-server#9377

It would helpful to have it returned in the results :)

Why

For open-prices

• Clarify the product quantity unit (g or mL) open-prices#115
• Barcode scanner (manual): allow search with wildcards with the help of search-a-licious ? open-prices-frontend#1235

[alexgarel]

@raphodn this is exactly the next step we need to do: make a list of all fields we need in search keeping a good balance between performance / completeness.