Create Zendesk custom object lookup relationship fields and query records by both forward and reverse relationship direction

Steps

1. Create a lookup field via POST /api/v2/custom_objects/{custom_object_key}/fields (or /api/v2/ticket_fields, /api/v2/user_fields, /api/v2/organization_fields) with type "lookup" and relationship_target_type set to a value like "zen:user", "zen:ticket", "zen:organization", or "zen:custom_object:{key}" (immutable after creation)
2. Create a custom object record and set the lookup value via POST /api/v2/custom_objects/{custom_object_key}/records, referencing the target by id, external_id ("external_id:ABC"), or a unique name ("name:car1")
3. List/read records with cursor pagination via GET /api/v2/custom_objects/{custom_object_key}/records (max 100 per page)
4. Query the relationship in reverse (find source records pointing at a target) via GET /api/v2/{target_type}/{target_id}/relationship_fields/{field_id}/{source_type}
5. Filter on lookup, date, or dropdown field values using structured search: POST /api/v2/custom_objects/{custom_object_key}/records/search with a JSON filter body ($eq/$and/$or, custom_object_fields.{field_key} namespacing)