Fix: attempt ood in spring

This change to address some of Olivier's comment on https://gitlab.inria.fr/stemcovid19/tac-server/backend-server/-/merge_requests/5

And try some OOD pattern respecting the Spring idioms.

Mostly:

• removed and replaced inline a 'wrapping service' who had a static method (not a Spring idiom)
• use of a mapper in VO for generating correct tokens (static or dynamic)
• double dispatch to avoid if regarding token types and be more object oriented (thanks @x-ctac)
• moved enum from service to vo
• switched from stream to for (just a matter of taste, we could fall back to stream later*)