`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.
server/src/main/java/org/elasticsearch/cluster/SnapshotsInProgress.java
long startTime,
        long repositoryStateId,
        Map<ShardId, ShardSnapshotStatus> shards,
        Map<String, Object> userMetadata,
        Version version,
        List<SnapshotFeatureInfo> featureStates
    ) {
        return Entry.snapshot(
            snapshot,
            includeGlobalState,
            partial,
            completed(shards.values()) ? State.SUCCESS : State.STARTED,
            indices,
            dataStreams,
            featureStates,
            startTime,
            repositoryStateId,
            shards,
            null,
            userMetadata,
            version
        );
    }
    /**
     * Creates the initial snapshot clone entry
     *
     * @param snapshot snapshot to clone into
     * @param source   snapshot to clone from
     * @param indices  indices to clone
     * @param startTime start time