relation abstract method
GET /{resource}/{id}/relations/{relation} — one relation manager's child
rows, in the same {data, meta} envelope list uses. The same read
path as list against a sibling URL, so it is fetched the same way:
through this data source, never a closure a host has to hand-wire.
search/sort/direction (P11) mirror list's exactly: the endpoint
answers them with the resource index's contract — an unknown sort key is
a 422, and a declared default sort applies when sort is absent.
Implementation
Future<PaginatedRecords> relation(
String resourceKey,
Object id,
RelationDescriptor relation, {
int page,
String? search,
String? sort,
String? direction,
});