This patch is only based on https://github.com/matrix-org/sliding-sync/ pull/304 since the specification wasn't updated. I'm basically guessing what a request and a response _should_ look like: * `RoomSubscription` gains a new field: `include_heros`, * `SlidingSyncRoom` gains a new field: `heros`, * `SlidingSyncRoomHero` is a new type.