### 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/cluster/metadata/ShutdownShardMigrationStatus.java
private final SingleNodeShutdownMetadata.Status status;
    private final long shardsRemaining;
    @Nullable
    private final String explanation;
    @Nullable
    private final ShardAllocationDecision allocationDecision;
    public ShutdownShardMigrationStatus(SingleNodeShutdownMetadata.Status status, long shardsRemaining) {
        this(status, shardsRemaining, null, null);
    }
    public ShutdownShardMigrationStatus(SingleNodeShutdownMetadata.Status status, long shardsRemaining, @Nullable String explanation) {
        this(status, shardsRemaining, explanation, null);
    }
    public ShutdownShardMigrationStatus(
        SingleNodeShutdownMetadata.Status status,
        long shardsRemaining,
        @Nullable String explanation,
        @Nullable ShardAllocationDecision allocationDecision
    ) {
        this.status = Objects.requireNonNull(status, "status must not be null");
        this.shardsRemaining = shardsRemaining;
        this.explanation = explanation;
        this.allocationDecision = allocationDecision;
    }
    public ShutdownShardMigrationStatus(StreamInput in) throws IOException {
        this.status = in.readEnum(SingleNodeShutdownMetadata.Status.class);
        this.shardsRemaining = in.readLong();
        this.explanation = in.readOptionalString();
        if (in.getTransportVersion().onOrAfter(ALLOCATION_DECISION_ADDED_VERSION)) {
            this.allocationDecision = in.readOptionalWriteable(ShardAllocationDecision::new);
        } else {