Add support for MSC3946: Dynamic room predecessor

[andybalaam]

MSC3946 enables us to specify the predecessor room using a new state event instead of the create event.

This doesn't have much effect on the sliding sync proxy, but it does affect the list of rooms that are returned under certain circumstances.

As I understand it, predecessor rooms should normally be excluded from the list of returned rooms, so the proxy will need to determine the predecessor using the new state event if present. I believe this will be needed if the include_old_rooms filter is active.

I implemented this stuff in matrix-js-sdk - happy to help if you have any questions. The most relevant code is probably in findPredecessor in room-state.ts.

[DMRobertson]

We do similar things for m.room.create as part of supporting the include_old_rooms flag. It should be easy to replicate the lookup.

The problem is that we had assumed that predecessors never change. But now it can. Will have to update some SQL and also the caches.

Relevant source:
https://github.com/matrix-org/sliding-sync/blob/main/state/rooms_table.go#L15
https://github.com/matrix-org/sliding-sync/blob/main/state/rooms_table.go#L49 need to be more selective about what "latches on"