Sliding sync: Add connection tracking to the `account_data` extension (#17695)

This is basically exactly the same logic as for receipts. Essentially we
just need to track which room account data we have and haven't sent down
to clients, and use that when we pull stuff out.

I think this just needs a couple of extra tests written

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Co-authored-by: Eric Eastwood <eric.eastwood@beta.gouv.fr>