### 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
)
        );
    }
    @Override
    protected ClusterBlockException checkBlock(StartDatafeedAction.Request request, ClusterState state) {
        
        
        
        return state.blocks().globalBlockedException(ClusterBlockLevel.METADATA_WRITE);
    }
    private void waitForDatafeedStarted(
        String taskId,
        StartDatafeedAction.DatafeedParams params,
        ActionListener<NodeAcknowledgedResponse> listener
    ) {
        DatafeedPredicate predicate = new DatafeedPredicate();
        persistentTasksService.waitForPersistentTaskCondition(
            taskId,
            predicate,
            params.getTimeout(),
            new PersistentTasksService.WaitForPersistentTaskListener<StartDatafeedAction.DatafeedParams>() {
                @Override
                public void onResponse(PersistentTasksCustomMetadata.PersistentTask<StartDatafeedAction.DatafeedParams> persistentTask) {
                    if (predicate.exception != null) {
                        
                        
                        cancelDatafeedStart(persistentTask, predicate.exception, listener);
                    } else {
                        listener.onResponse(new NodeAcknowledgedResponse(true, predicate.node));
                    }