Make SnapshotsInProgress diffable #88732
Description
SnapshotsInProgress can be pretty large and today it's an AbstractNamedDiffable so any changes to any snapshot status result in sending the whole thing over the wire to all nodes. Yet, it undergoes a lot of small changes as snapshots proceed towards completion. We should make it properly diffable so that small changes have smaller representations on the wire and unchanged parts can be shared between instances.
Relates #77466.