`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
RepositoryShardId shardId = shardEntry.getKey();
                        writeShardSnapshotStatus(builder, shardId.index(), shardId.shardId(), shardEntry.getValue());
                    }
                }
                builder.endArray();
            }
            builder.array("data_streams", dataStreams.toArray(new String[0]));
            builder.endObject();
            return builder;
        }
        private static void writeShardSnapshotStatus(XContentBuilder builder, ToXContent indexId, int shardId, ShardSnapshotStatus status)
            throws IOException {
            builder.startObject();
            builder.field("index", indexId);
            builder.field("shard", shardId);
            builder.field("state", status.state());
            builder.field("generation", status.generation());
            builder.field("node", status.nodeId());
            if (status.state() == ShardState.SUCCESS) {
                final ShardSnapshotResult result = status.shardSnapshotResult();
                builder.startObject("result");
                builder.field("generation", result.getGeneration());
                builder.humanReadableField("size_in_bytes", "size", result.getSize());
                builder.field("segments", result.getSegmentCount());
                builder.endObject();
            }
            if (status.reason() != null) {
                builder.field("reason", status.reason());
            }
            builder.endObject();