Create a dev mode option to test arbitrary versions of the API

[teolemon]

• Story: Create a dev mode option to test arbitrary versions of the API
• ref: https://openfoodfacts.github.io/openfoodfacts-server/api/ref-api-and-product-schema-change-log/
• Needs Dart work before

[monsieurtanuki]

@teolemon I already coded a couple of PRs adding flexibility to Product in off-dart, which is definitely a requirement for the current issue.
Both PRs were ignored, and after 2 months I just closed them.
Is there a reason why a new PR wouldn't be ignored as well this time?