Allow setting prepared nutrition data through API

[stephanegigandet]

Related to #1151 which was implemented only for the web product edit form, and not for the API.

[njean42]

I see basically two ways of doing this:

1. add an extra parameter, like prepared=1;
2. add _prepared to every nutriment_** parameter, as well as to nutrition_data_per (= 100g_prepared or serving_prepared), which would look like a product's JSON.

I guess the first option is easier/clearer, but I don't know my way around server internals.

[github-actions]

This issue has been open 90 days with no activity. Can you give it a little love by linking it to a parent issue, adding relevant labels and projets, creating a mockup if applicable, adding code pointers from https://github.com/openfoodfacts/openfoodfacts-server/blob/main/.github/labeler.yml, giving it a priority, editing the original issue to have a more comprehensive description… Thank you very much for your contribution to 🍊 Open Food Facts