### 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
);
        assertThat(e.getResponse().getStatusLine().getStatusCode(), equalTo(404));
        assertThat(e.getMessage(), containsString("No known job with id 'non-existing-job'"));
    }
    public void testGetJob_GivenJobExists() throws Exception {
        createFarequoteJob("get-job_given-job-exists-job");
        Response response = client().performRequest(
            new Request("GET", MachineLearning.BASE_PATH + "anomaly_detectors/get-job_given-job-exists-job/_stats")
        );
        String responseAsString = EntityUtils.toString(response.getEntity());
        assertThat(responseAsString, containsString("\"count\":1"));
        assertThat(responseAsString, containsString("\"job_id\":\"get-job_given-job-exists-job\""));
    }
    public void testGetJobs_GivenSingleJob() throws Exception {
        String jobId = "get-jobs_given-single-job-job";
        createFarequoteJob(jobId);
        
        String explictAll = EntityUtils.toString(
            client().performRequest(new Request("GET", MachineLearning.BASE_PATH + "anomaly_detectors/_all")).getEntity()
        );
        assertThat(explictAll, containsString("\"count\":1"));
        assertThat(explictAll, containsString("\"job_id\":\"" + jobId + "\""));
        
        String implicitAll = EntityUtils.toString(
            client().performRequest(new Request("GET", MachineLearning.BASE_PATH + "anomaly_detectors")).getEntity()
        );
        assertThat(implicitAll, containsString("\"count\":1"));
        assertThat(implicitAll, containsString("\"job_id\":\"" + jobId + "\""));
    }
    public void testGetJobs_GivenMultipleJobs() throws Exception {