Spaces hierarchy lacks a way to know which servers to join via

**Link to problem area**:

https://spec.matrix.org/v1.15/client-server-api/#get_matrixclientv1roomsroomidhierarchy

**Issue**

`SpaceHierarchyRoomsChunk` lacks a `via` key with the servers to join the given room via. This means clients have to use other means to get server names e.g looking at the `sender` key of `children_state` events.

Start with the linked Matrix Client-Server API section for GET /_matrix/client/v1/rooms/{roomId}/hierarchy and inspect the definition of SpaceHierarchyRoomsChunk. Determine how a via server list should be represented for each room, then update the specification so clients can obtain the servers needed to join; done means the response schema and behavior are unambiguous.

Written by the indexing model from the issue text.