de-MSC-ifying space summaries (MSC2946) (#3134)

- This PR moves and refactors the
[code](https://github.com/matrix-org/dendrite/blob/main/setup/mscs/msc2946/msc2946.go)
for
[MSC2946](https://github.com/matrix-org/matrix-spec-proposals/pull/2946)
('Space Summaries') to integrate it into the rest of the codebase.
- Means space summaries are no longer hidden behind an MSC flag
- Solves #3096

Signed-off-by: Sam Wedgwood <sam@wedgwood.dev>

internal/caching/cache_roomservernids.go

@@ -8,6 +8,7 @@ type RoomServerCaches interface {
 	RoomServerNIDsCache
 	RoomVersionCache
 	RoomServerEventsCache
+	RoomHierarchyCache
 	EventStateKeyCache
 	EventTypeCache
 }