BibField: new @persistent_identifier "decorator"

* New "decorator" inside the config file that allows to specify
  when a json key is used as persistent identifier and its order
  of preference.  All json keys that should be considered as persistent
  identifier are stored inside the calculated field
  _persistent_identifiers_keys.  (closes #1500)

* New getter to provide a subset of the record containing all
  the persistent identifiers, based on _persisitent_identifiers_keys

Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@cern.ch>
Tested-by: Tibor Simko <tibor.simko@cern.ch>