BibField: CFG_BIBFORMAT_HIDDEN_RECJSON_FIELDS

* Introduces ad-interim new config variable
  CFG_BIBFORMAT_HIDDEN_RECJSON_FIELDS to list which recjson fields
  need to be not exposed via public interfaces.
  (addresses #2197)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>