### 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
putJob(compositeJob);
        openJob(compositeJobId);
        assertBusy(() -> assertEquals(getJobStats(compositeJobId).get(0).getState(), JobState.OPENED));
        AggregatorFactories.Builder aggs = new AggregatorFactories.Builder();
        aggs.addAggregator(
            AggregationBuilders.composite(
                "buckets",
                Collections.singletonList(
                    new DateHistogramValuesSourceBuilder("timebucket").fixedInterval(new DateHistogramInterval("1h")).field("time")
                )
                
            ).subAggregation(AggregationBuilders.max("time").field("time")).size(1)
        );
        DatafeedConfig compositeDatafeedConfig = createDatafeedBuilder(
            compositeJobId + "-datafeed",
            compositeJobId,
            Collections.singletonList(indexName)
        ).setParsedAggregations(aggs).setFrequency(TimeValue.timeValueHours(1)).build();
        putDatafeed(compositeDatafeedConfig);
        startDatafeed(compositeDatafeedConfig.getId(), 0L, null);
        
        assertBusy(() -> assertThat(getDataCounts(compositeJobId).getProcessedRecordCount(), greaterThan(0L)));
        stopDatafeed(compositeDatafeedConfig.getId());
        assertBusy(() -> assertThat(getJobStats(compositeJobId).get(0).getState(), is(oneOf(JobState.CLOSED, JobState.OPENED))));
        
        if (getJobStats(compositeJobId).get(0).getState().equals(JobState.OPENED)) {
            aggs = new AggregatorFactories.Builder();
            aggs.addAggregator(
                AggregationBuilders.composite(
                    "buckets",