Use hosts in rooms of the hierarchy for via parameter

[MatMaul]

Pull Request Checklist

Scenarii to reproduce:

• user A on server A creates a space
• user A creates a room in this space, with join restricted to the members of the space
• user B on server B joins the space and the room
• user A leaves the space and the room
• user AB on server A joins the space
• user AB can't see the room in the roomlist of the space
• if an user on server A joins the room, the problem is solved

What is happening is that there is no valid via anymore for the room: it is not avail anymore on its original homeserver A, which is the only via available in the child event.

This PR gathers via parameters of all rooms of the hierarchy previously handled, and pass it to the fed endpoint when trying to fetch the remote hierarchy.

• Pull request is based on the develop branch
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• Code style is correct
  (run the linters)

[kfiven]

Hi, I feel like you are trying to solve the same problem as in #18070 ?

[MatMaul]

Indeed. I am not sure they are contradictory however.

This change would still help if no valid via is passed by the client.

Opinions welcome!