Return allowed_room_ids in the client /hierarchy response

[barodeur]

Part of #18731 (Support Matrix 1.15).

Since Matrix 1.15, the spec defines allowed_room_ids on the room summaries returned by GET /_matrix/client/v1/rooms/{roomId}/hierarchy, so that clients can tell whether a restricted room can be joined or only knocked at:

allowed_room_ids — [Room ID] — If the room is a restricted room, these are the room IDs which are specified by the join rules. Empty or omitted otherwise. Added in v1.15

— Matrix Spec

Synapse currently strips the field from the client /hierarchy response before returning it — a guard added in matrix-org/synapse#12175 (2022), correct at the time, when the spec defined the field for federation only and it was leaking into client responses. The other two surfaces that define the field (/room_summary and the federation /hierarchy) already return it.

This PR removes the strip in _RoomEntry.as_json (and the now-unused for_client parameter), so client hierarchy entries include allowed_room_ids for restricted rooms, both local and received over federation. The first commit adds the failing test coverage, the second removes the strip.

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