### 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
this.startTime = params.getStartTime();
            this.endTime = params.getEndTime();
        }
        public String getDatafeedId() {
            return datafeedId;
        }
        public long getDatafeedStartTime() {
            return startTime;
        }
        @Nullable
        public Long getEndTime() {
            return endTime;
        }
        public boolean isLookbackOnly() {
            return endTime != null;
        }
        /**
         * Set the datafeed runner <em>if</em> the task has not already been told to stop or isolate.
         * @return A {@link StoppedOrIsolated} object that indicates whether the
         *         datafeed task had previously been told to stop or isolate.  {@code datafeedRunner}
         *         will only be set to the supplied value if the return value of this method is
         *         {@link StoppedOrIsolated#NEITHER}.
         */
        StoppedOrIsolated setDatafeedRunner(DatafeedRunner datafeedRunner) {
            return executeIfNotStoppedOrIsolated(() -> this.datafeedRunner = Objects.requireNonNull(datafeedRunner));
        }
        /**
         * Run a command <em>if</em> the task has not already been told to stop or isolate.
         * @param runnable The command to run.
         * @return A {@link StoppedOrIsolated} object that indicates whether the datafeed task