### 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/src/main/java/org/elasticsearch/xpack/ml/action/TransportStartDatafeedAction.java
public enum StoppedOrIsolated {
            NEITHER,
            ISOLATED,
            STOPPED
        }
        private final String datafeedId;
        private final long startTime;
        private final Long endTime;
        /**
         * This must always be set within a synchronized block that also checks
         * the value of the {@code stoppedOrIsolatedBeforeRunning} flag.
         */
        private DatafeedRunner datafeedRunner;
        private StoppedOrIsolated stoppedOrIsolated = StoppedOrIsolated.NEITHER;
        DatafeedTask(
            long id,
            String type,
            String action,
            TaskId parentTaskId,
            StartDatafeedAction.DatafeedParams params,
            Map<String, String> headers
        ) {
            super(id, type, action, "datafeed-" + params.getDatafeedId(), parentTaskId, headers);
            this.datafeedId = params.getDatafeedId();