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)

* Removes undocumented 'fields' argument in bibfield.get_record()
  which was making get_record() returning a different object type
  depending on having 0 or more values in fields.

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>