Add federation /versions endpoint (MSC3723)

[ShadowJonathan]

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This implements https://github.com/matrix-org/matrix-doc/pull/3723.

Pull Request Checklist

• [x] Pull request is based on the develop branch
• [x] Pull request includes a changelog file. The entry should:
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Signed-off-by: Jonathan de Jong <[email protected]>

[erikjohnston]

@ShadowJonathan I assume you aren't looking to have it merged until the MSC progresses, right?

[ShadowJonathan]

Yes, at the moment the MSC is blocked until the endpoint sees good application (probably in conjunction with another MSC), so sorry this'll probably bitrot for a while.

That said, i believe that its small scale means it can be easily resurrected once interest on the MSC increases. I'll put it in draft until such a time comes.