`feature_ref` should we introduce cleabs to be able to link with other services

[esgn]

feature_ref should we introduce cleabs to be able to link with other services

@LionelZoubritzky-IGN if I try summarize:

• Only certain datasets will have cleabs
• reverse geocoding https://data.geopf.fr/geocodage/openapi#reverse may return extrafields > cleabs for index=poi
• The idea would be to be able to reuse this cleabs
• gpf_wfs_get_features with an attribute filter on cleabs allows it
• Do we need something simpler ?

Tentative conclusion

The good idea would be to return a feature_ref instead of a cleabs when a service different from WFS returns it. In order to do so we need this new feature ignfab/gpf-schema-store#60