List searchable applicant fields

Returns the catalog of applicant search fields available to the authenticated user for use in `ApplicantSearchRequest.fields[].field` or `ApplicantSearchRequest.orderBy`. The catalog includes both system-defined fields and the company's user-defined fields (UDFs), each with its data type, optional grouping, multi-valued flag, and returnable flag. Every entry in this catalog is searchable — reference it in `fields` criteria or `orderBy`. Whether it can also be returned depends on its `returnable` flag: - **`returnable: true`** — can be listed in `returnFields`; its value is populated on each hit. - **`returnable: false`** — indexed for search, but not stored in the search index, so it cannot be returned.