Extract server from roomId to get hierarchy over federation

[kfiven]

Fixes remaining parts of #17143

The change actually need via parameter in the /hierarchy endpoint in spec, but IMO temporarily its a good fix to at least look for the server extracting from the room id rather than nothing.

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[anoadragon453]

Thanks for making this PR. I'm afraid that domains in room IDs are really intended just for namespacing, and are not meant to be extracted and used for other purposes.

As you say, the real fix is to include the domain in the via parameters. While this fix would help with some cases in the wild, domains in room IDs aren't meant to be used in that way.