### Description

ShutdownShardMigrationStatus serializes ShardAllocationDecision, which, as of #97803, is chunked. ShardAllocationDecision grows with the number of nodes. Therefore, it would save heap space to chunk ShutdownShardMigrationStatus as well.
server/src/main/java/org/elasticsearch/common/xcontent/ChunkedToXContentHelper.java
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0 and the Server Side Public License, v 1; you may not use this file except
 * in compliance with, at your election, the Elastic License 2.0 or the Server
 * Side Public License, v 1.
 */
package org.elasticsearch.common.xcontent;
import org.elasticsearch.common.collect.Iterators;
import org.elasticsearch.xcontent.ToXContent;
import java.util.Iterator;
import java.util.Map;
import java.util.function.Function;
public enum ChunkedToXContentHelper {
    ;
    public static Iterator<ToXContent> startObject() {
        return Iterators.single(((builder, params) -> builder.startObject()));
    }
    public static Iterator<ToXContent> startObject(String name) {
        return Iterators.single(((builder, params) -> builder.startObject(name)));
    }
    public static Iterator<ToXContent> endObject() {
        return Iterators.single(((builder, params) -> builder.endObject()));
    }