### 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
indexDocs(logger, "data", numDocs, now.minus(Duration.ofDays(14)).toEpochMilli(), now.toEpochMilli());
        Job.Builder job = createScheduledJob("lookback-job-query-delay-updated");
        putJob(job);
        String datafeedId = "lookback-datafeed-query-delay-updated";
        DatafeedConfig datafeedConfig = createDatafeed(datafeedId, job.getId(), Collections.singletonList("data"));
        putDatafeed(datafeedConfig);
        openJob(job.getId());
        assertBusy(() -> assertEquals(getJobStats(job.getId()).get(0).getState(), JobState.OPENED));
        
        assertDatafeedStats(datafeedId, DatafeedState.STOPPED, job.getId(), equalTo(0L));
        startDatafeed(datafeedId, 0L, now.toEpochMilli());
        assertBusy(() -> {
            assertThat(getDataCounts(job.getId()).getProcessedRecordCount(), equalTo(numDocs));
            
            assertDatafeedStats(datafeedId, DatafeedState.STOPPED, job.getId(), greaterThan(0L));
        }, 60, TimeUnit.SECONDS);
        waitUntilJobIsClosed(job.getId());
        
        updateDatafeed(new DatafeedUpdate.Builder(datafeedId).setQueryDelay(TimeValue.timeValueSeconds(777)).build());
        
        assertDatafeedStats(datafeedId, DatafeedState.STOPPED, job.getId(), greaterThan(0L));
    }
    public void testStopAndRestartCompositeDatafeed() throws Exception {
        client().admin()
            .cluster()
            .prepareUpdateSettings()
            .setPersistentSettings(Settings.builder().put("logger.org.elasticsearch.xpack.ml.datafeed", "TRACE").build())
            .get();
        String indexName = "stop-restart-data";
        client().admin().indices().prepareCreate("stop-restart-data").setMapping("time", "type=date").get();