Correctly changes to required state config in sliding sync (#17785)

Fixes https://github.com/element-hq/synapse/issues/17698



This handles `required_state` changes by checking if new state has been
added to the config, and if so fetching and returning that from the
current state.

This also takes care to ensure that given a state entry S that is added,
removed and then re-added that we do *not* send S down a second time if
there have been no changes to S in the current state. This is fine for
Rust SDK (as it just remembers all state), but we might decide not to do
this behaviour in the MSC. If we decide to always send down S then its
easy enough to rip out all the code.

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