### 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
RestStatus.CONFLICT
                        );
                    }
                    listener.onFailure(e);
                }
            };
        
        Consumer<Job> createDataExtractor = job -> {
            final List<String> remoteIndices = RemoteClusterLicenseChecker.remoteIndices(params.getDatafeedIndices());
            if (remoteIndices.isEmpty() == false) {
                final RemoteClusterLicenseChecker remoteClusterLicenseChecker = new RemoteClusterLicenseChecker(
                    client,
                    MachineLearningField.ML_API_FEATURE
                );
                remoteClusterLicenseChecker.checkRemoteClusterLicenses(
                    RemoteClusterLicenseChecker.remoteClusterAliases(
                        transportService.getRemoteClusterService().getRegisteredRemoteClusterNames(),
                        params.getDatafeedIndices()
                    ),
                    ActionListener.wrap(response -> {
                        if (response.isSuccess() == false) {
                            listener.onFailure(createUnlicensedError(params.getDatafeedId(), response));
                        } else if (remoteClusterClient == false) {
                            listener.onFailure(
                                ExceptionsHelper.badRequestException(
                                    Messages.getMessage(
                                        Messages.DATAFEED_NEEDS_REMOTE_CLUSTER_SEARCH,
                                        datafeedConfigHolder.get().getId(),
                                        RemoteClusterLicenseChecker.remoteIndices(datafeedConfigHolder.get().getIndices()),
                                        clusterService.getNodeName()