### 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
createFarequoteJob("given-multiple-jobs-job-1");
        createFarequoteJob("given-multiple-jobs-job-2");
        createFarequoteJob("given-multiple-jobs-job-3");
        
        String explicitAll = EntityUtils.toString(
            client().performRequest(new Request("GET", MachineLearning.BASE_PATH + "anomaly_detectors/_all")).getEntity()
        );
        assertThat(explicitAll, containsString("\"count\":3"));
        assertThat(explicitAll, containsString("\"job_id\":\"given-multiple-jobs-job-1\""));
        assertThat(explicitAll, containsString("\"job_id\":\"given-multiple-jobs-job-2\""));
        assertThat(explicitAll, containsString("\"job_id\":\"given-multiple-jobs-job-3\""));
        
        String implicitAll = EntityUtils.toString(
            client().performRequest(new Request("GET", MachineLearning.BASE_PATH + "anomaly_detectors")).getEntity()
        );
        assertThat(implicitAll, containsString("\"count\":3"));
        assertThat(implicitAll, containsString("\"job_id\":\"given-multiple-jobs-job-1\""));
        assertThat(implicitAll, containsString("\"job_id\":\"given-multiple-jobs-job-2\""));
        assertThat(implicitAll, containsString("\"job_id\":\"given-multiple-jobs-job-3\""));
    }
    
    public void testUsage() throws IOException {
        createFarequoteJob("job-1");
        createFarequoteJob("job-2");
        Map<String, Object> usage = entityAsMap(client().performRequest(new Request("GET", "_xpack/usage")));
        assertEquals(2, XContentMapValues.extractValue("ml.jobs._all.count", usage));
        assertEquals(2, XContentMapValues.extractValue("ml.jobs.closed.count", usage));
        openJob("job-1");
        usage = entityAsMap(client().performRequest(new Request("GET", "_xpack/usage")));
        assertEquals(2, XContentMapValues.extractValue("ml.jobs._all.count", usage));