### 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
return new DatafeedNodeSelector(
                clusterState,
                resolver,
                params.getDatafeedId(),
                params.getJobId(),
                params.getDatafeedIndices(),
                params.getIndicesOptions()
            ).selectNode(candidateNodes);
        }
        @Override
        public void validate(StartDatafeedAction.DatafeedParams params, ClusterState clusterState) {
            new DatafeedNodeSelector(
                clusterState,
                resolver,
                params.getDatafeedId(),
                params.getJobId(),
                params.getDatafeedIndices(),
                params.getIndicesOptions()
            ).checkDatafeedTaskCanBeCreated();
        }
        @Override
        protected void nodeOperation(
            final AllocatedPersistentTask allocatedPersistentTask,
            final StartDatafeedAction.DatafeedParams params,
            final PersistentTaskState state
        ) {
            DatafeedTask datafeedTask = (DatafeedTask) allocatedPersistentTask;
            DatafeedState datafeedState = (DatafeedState) state;