List of filterable fields to return filter options for. Valid values: asset_type, asn, custom_tags, added_to_project, discovered, apex, ip_geo_country_iso, ip_owner, static_asset, registry, referenced_ip, exposure_id, severity, asset_exposure_score, last_scanned_at, open_port_number, open_port_service, open_port_protocol, waf_detected, waf_name, technology_name, is_responsive, certificate_issuer, certificate_expires_at, certificate_issued_at. You can also use all to return all available filterable fields.