### Description

This applies to the following APIs:
- [x] open anomaly detection
- [x] start datafeed
- [x] start data frame analytics job
- [x] start trained model deployment

Currently, if the operation takes longer than the configured `timeout`, the response status code is `500`.

With Kibana in mind, a `4xx` response would be more suitable as it will lead to showing more useful information to the user about why the request failed.

Looking at [RFC 9110](https://www.rfc-editor.org/rfc/rfc9110.html#name-408-request-timeout), it seems the most appropriate `4xx` code would be `408 Request Timeout`, even though the RFC describes a different situation. 

The alternative, `504 Gateway Timeout`, would be more appropriate for this situation but it wouldn't improve the user experience in Kibana.
x-pack/plugin/ml/qa/native-multi-node-tests/src/javaRestTest/java/org/elasticsearch/xpack/ml/integration/MlJobIT.java
import org.elasticsearch.client.Response;
import org.elasticsearch.client.ResponseException;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.common.util.concurrent.ConcurrentCollections;
import org.elasticsearch.common.util.concurrent.ThreadContext;
import org.elasticsearch.common.xcontent.support.XContentMapValues;
import org.elasticsearch.test.SecuritySettingsSourceField;
import org.elasticsearch.test.rest.ESRestTestCase;
import org.elasticsearch.xpack.core.ml.MlTasks;
import org.elasticsearch.xpack.core.ml.integration.MlRestTestStateCleaner;
import org.elasticsearch.xpack.core.ml.job.config.Job;
import org.elasticsearch.xpack.core.ml.job.persistence.AnomalyDetectorsIndex;
import org.elasticsearch.xpack.core.ml.job.persistence.AnomalyDetectorsIndexFields;
import org.elasticsearch.xpack.core.ml.job.process.autodetect.state.TimingStats;
import org.elasticsearch.xpack.core.security.authc.support.UsernamePasswordToken;
import org.elasticsearch.xpack.ml.MachineLearning;
import org.junit.After;
import java.io.IOException;
import java.util.Collections;
import java.util.Locale;
import java.util.Map;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReference;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import static org.hamcrest.Matchers.containsString;
import static org.hamcrest.Matchers.equalTo;
import static org.hamcrest.Matchers.hasEntry;
import static org.hamcrest.Matchers.not;
public class MlJobIT extends ESRestTestCase {