Add field contributor_orgs.name_identifier and contributor_orgs.name_identifier_type

contributor_orgs.name_identifier must have a corresponding contributor.name_identifier_type. There must be checked on the backend, no value given, then use the string "N.A."

Start by locating the contributor_orgs and contributor models or schemas and the backend validation entry point. Confirm how missing values are represented, then ensure the two fields and their corresponding-value rule are covered by the relevant backend tests, with "N.A." used when no value is given.

Written by the indexing model from the issue text.