Move interfaces and related data model to a separate subproject

Custom fields like other modules have several API interfaces and quite complex data model. Right now both of them included into a single artifact along with the implementation. This is OK if custom fields embedded into a module completely, like in case of mod-users. But it's not optimal when only API model is required, like in case of mod-user-import which has to include CF's json schemas as part of its own schemas to make the model available (alternative is to import the whole folio-custom-fields.jar ).

It is proposed to separate API from the rest and have independent API artifact which can be imported by a module. The artifact should include API interfaces and required data model only. It's going to be called folio-custom-fields-api. Structure I should be a separate maven subproject if possible