List valid workgroups

Returns every workgroup the authenticated user is permitted to assign, valid for `POST`/`DELETE` `/api/v1/requirements/{guid}/workgroups/{workgroupId}`. This list is user-scoped, not just company-scoped — a value returned here is always accepted as `workgroupId` on that endpoint for this same user, but the list (and therefore what's valid) may differ between users.