Reads up to 100 relation types in one call, for the rules list and the rule editor. POST rather than GET because the request carries a list of ids in a body; it is a read and requires read permission only. Every id sent comes back, with defaults for the ones the Rules Engine has no record of, so match on relationId rather than on position. Unlike the single read, this endpoint does not check the ids against PIM and so never returns 404.