### 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/DatafeedJobsIT.java
import org.elasticsearch.xpack.core.ml.job.results.Bucket;
import org.elasticsearch.xpack.core.ml.utils.Intervals;
import org.hamcrest.Matcher;
import org.junit.After;
import java.time.Duration;
import java.time.Instant;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicReference;
import static org.elasticsearch.xpack.ml.support.BaseMlIntegTestCase.createDatafeed;
import static org.elasticsearch.xpack.ml.support.BaseMlIntegTestCase.createDatafeedBuilder;
import static org.elasticsearch.xpack.ml.support.BaseMlIntegTestCase.createScheduledJob;
import static org.elasticsearch.xpack.ml.support.BaseMlIntegTestCase.getDataCounts;
import static org.elasticsearch.xpack.ml.support.BaseMlIntegTestCase.indexDocs;
import static org.hamcrest.Matchers.equalTo;
import static org.hamcrest.Matchers.greaterThan;
import static org.hamcrest.Matchers.hasSize;
import static org.hamcrest.Matchers.is;
import static org.hamcrest.Matchers.oneOf;
public class DatafeedJobsIT extends MlNativeAutodetectIntegTestCase {
    @After
    public void cleanup() {
        client().admin()
            .cluster()
            .prepareUpdateSettings()
            .setPersistentSettings(Settings.builder().putNull("logger.org.elasticsearch.xpack.ml.datafeed").build())