Add flexibility to PriceTotalStats

[monsieurtanuki]

Why - Problem description

Currently in off-dart, we support less than 20 fields for PriceTotalStats.
And there are more than 50 fields on the server (https://prices.openfoodfacts.org/api/v1/stats).
In order to add the fields we would need to code, push the code, have it reviewed and merged and released.
That's due to the way it was coded, relying on JsonSerializable.

What - Proposed solution

A flexible solution would be to keep the JSON and just let the users get what they want.
Something like int? getInt(String tag).
We may also provide a list of static const String typical tags. If there's a new tag, we wouldn't need to release again: the users would be able to query that new tag regardless.

Additional context

cf. FlexibleProduct in #1056.
cf. openfoodfacts/smooth-app#6128
cf. openfoodfacts/smooth-app#6553

[AshutoshKhadse23]

Hello @monsieurtanuki,
I've opened a PR that addresses all the checks mentioned in the issue and is ready for review.

[monsieurtanuki]

@AshutoshKhadse23 For the record, you're supposed to ask to be assigned first.

[AshutoshKhadse23]

Sorry for that but could you assign this to me !