Expose an API route to expose Robotoff predictions for nutrition facts

[teolemon]

Problem

• Expose an API route to expose Robotoff predictions for nutrition facts
• The data is currently stored in the Prediction table.
• A new route should be created to return the best prediction for a given product.

Related to

• Offer in-line validation of Robotoff predictions for nutrition facts smooth-app#3278
• Use Robotoff to prefill some nutrition fields hunger-games#74

Part of

• Nutrition (tracker) #372

[raphael0202]

We can already use the /predictions route to get all nutrient predictions associated with a product:

https://robotoff.openfoodfacts.net/api/v1/predictions?types=nutrient&barcode=3017800502044