Isolate schemas (railjson)

[flomonster]

Short description meaningful for devs and users

• [ ] Isolate and create a importable python module.
• [ ] Add the dependency to api and infra-generator

[multun]

Once the django API will be merged with the edition backend, the schema will be all that remains in the api python module. It will thus be isolated, and may be published on a package registry.