loadRelation method
One relation's rows for the record this provider holds — the same
read path load() uses, against a sibling URL. A pass-through, not a
state change: RelationSectionWidget owns its own loading/failure
state for it, the same division ResourceListProvider keeps between
itself and the screen that renders its pages.
Implementation
Future<PaginatedRecords> loadRelation(
RelationDescriptor relation, {
int page = 1,
}) => source.relation(resource.key, id, relation, page: page);