Restrict the rooms a guest can join

This issue has been migrated from [#10170](https://github.com/matrix-org/synapse/issues/10170).

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The GNOME community would like to be a good citizen in the Matrix universe, and not be an entrypoint for spam to other instances/room. To do so, we will restrict registration, but we would love to open guest accounts instead. **The main blocker is we can't prevent guest accounts from spamming other instances**.

Since rooms don't belong to a single server but are distributed in the Matrix universe, it's not possible to develop a feature like "guest accounts should not be able to leave the server". **An flexible way to achieve what we want would be to allow guest accounts to join rooms _only_ if there already is a `:gnome.org` alias bound to this room** at the moment when they are trying to join. A less flexible (but maybe simpler?) way to do it would be to implement a room allow-list in `homeserver.yaml`. Per my understanding, this is something the homeserver of the guest account needs to enforce and doesn't require a specific MSC for that.