Add ISO3 code for countries

[monsieurtanuki]

Why - Problem description

In Smoothie we somehow need the 3-letter code for countries (e.g. 'FRA' for France).
Of course we could add it in Smoothie.
That would make sense to add it in the current project, though, so that other projects don't have to reinvent the wheel.

What - Proposed solution

We should add field String iso3code in OpenFoodFactsCountry and populate it.

Additional context

cf. openfoodfacts/smooth-app#6541

[pavanlashkari]

Hey @monsieurtanuki , would it be okay if I took care of this?