Get only the relationship fields of a custom object

Returns the same CustomObjectDetailsDto shape as getCustomObject, but customData is restricted to relationship reference keys only — scalar, dropdown, date, and other non-relationship fields are omitted. Each objectType.sections[*].fieldIds list is filtered to retain only the field IDs that correspond to relationship field definitions; empty sections are preserved so the UI gets a stable section layout.

Useful when a caller needs to display a CO's links to other entities without paying the cost of resolving the full custom_data payload.