This is so that we can cache it.
We also move the sliding sync types to
`synapse/types/handlers/sliding_sync.py`. This is mainly in-prep for
#17599 to avoid circular imports.
The only change in behaviour is that
`RoomSyncConfig.combine_sync_config(..)` now returns a new room sync
config rather than mutating in-place.
Reviewable commit-by-commit.
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Co-authored-by: Eric Eastwood <eric.eastwood@beta.gouv.fr>
