Refactoring: move Taxonomy methods to a specific file

[monsieurtanuki]

Why - Problem description

Our main file open_food_api_client.dart is very big (1291 lines).

What - Proposed solution

If we moved "taxonomy" methods to a specific file, it would go down to 1024 lines. Which is still too big, but it would already be a 20% decrease. And 1024 is 2^10, but I don't think we can maintain that geeky number of lines very long 😉

Additional context

That would be a breaking change.
Perhaps we would need to refactor the methods another way, when integrating obf opf opff (e.g. each with a specific "login" method or a common method?)