When using self-service mode and trying to link with a remote matrix
room (room not in the same HS as the bridge user), then we need to add
the "via" HSs to join the room with, or else it fails.

We get it from the "m.space.child" in the "children_state" of the space
hierarchy.

Co-authored-by: Cadence Ember <cadence@disroot.org>
Reviewed-on: #60
Co-authored-by: Elliu <elliu@hashi.re>
Co-committed-by: Elliu <elliu@hashi.re>

This _should_ be awaited all the way up, but it didn't work for me,
and better safe than sorry I guess?