List universal search fields

Returns every field indexed on the universal search core. All entries contribute to the aggregated text index that the `query` parameter matches against — per-field criteria are **not** supported on universal search; only the single `query` string is used. Only entries with `returnable: true` appear on `UniversalSearchHit`: - `entity_gk` - `guid` - `entity_name` - `entity_type_id` - `display_desc` - `score` - `entity_type_name` - `company_gk` The remaining entries are indexed but not stored.