The JSON marshalling was migrated to nuxeo-core-io. An enricher system is also available. See
org.nuxeo.ecm.core.io.marshallers.json.enrichers.BreadcrumbJsonEnricher for an example. To migrate an
existing enricher, keep the marshalling code and use it in class implementing
AbstractJsonEnricher<DocumentModel> (the use of contextual parameters is a bit different but
compatible / you have to manage the enricher's parameters yourself). Don't forget to contribute to
service org.nuxeo.ecm.core.io.registry.MarshallerRegistry to register your enricher.