[FEATURE] Implement AD TransportGetFieldMappingsAction

[dbwiddis]

Is your feature request related to a problem?

AbstractAnomalyDetectionActionHandler executes GetFieldMappingsAction twice (line 388 and 668) as part of field validation steps. The implementation involves cluster-level / shard-level actions that go beyond capabilities of the Rest client. These need to be investigated and implemented.

What solution would you like?

Update SDKClusterService to offer an API to accomplish the above.

[dbwiddis]

Reopening this issue as while the request has been added to the SDKClient, it has not yet been implemented in the AD AbstractAnomalyDetectionActionHandler class. (CC: @joshpalis can you fix?)

[dbwiddis]

https://github.com/opensearch-project/anomaly-detection/blob/697b9b7a50b5d1f22911135641180817fd06bf1d/src/main/java/org/opensearch/ad/rest/handler/AbstractAnomalyDetectorActionHandler.java#L384-L388

https://github.com/opensearch-project/anomaly-detection/blob/697b9b7a50b5d1f22911135641180817fd06bf1d/src/main/java/org/opensearch/ad/rest/handler/AbstractAnomalyDetectorActionHandler.java#L674-L678

In both cases the ActionListener has already been created, and would throw exceptions on failure if called, so it's mostly uncommenting out the client.execute() transport pieces, replacing with the new sdkClient REST call.