Add CompletionStage variants to methods in the Client Interface and default to ActionListener impl (opensearch-project#18998)

* Add CompletableFuture variables to methods in the Client Interface and default to ActionListener impl

Signed-off-by: Craig Perkins <cwperx@amazon.com>

* Add to CHANGELOG

Signed-off-by: Craig Perkins <cwperx@amazon.com>

* Fix typo in CHANGELOG

Signed-off-by: Craig Perkins <cwperx@amazon.com>

* Switch to CompletionStage

Signed-off-by: Craig Perkins <cwperx@amazon.com>

* Update CHANGELOG entry

Signed-off-by: Craig Perkins <cwperx@amazon.com>

---------

Signed-off-by: Craig Perkins <cwperx@amazon.com>

Author: cwperks

CHANGELOG.md

@@ -33,6 +33,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Remove `experimental` designation from transport-grpc settings ([#18915](https://github.com/opensearch-project/OpenSearch/pull/18915))
 - Rename package org.opensearch.plugin,transport.grpc to org.opensearch.transport.grpc ([#18923](https://github.com/opensearch-project/OpenSearch/pull/18923))
 - Added overload constructor for Translog to accept Channel Factory as a parameter ([#18918](https://github.com/opensearch-project/OpenSearch/pull/18918))
+- Add CompletionStage variants to methods in the Client Interface and default to ActionListener impl ([#18998](https://github.com/opensearch-project/OpenSearch/pull/18998))
 
 ### Fixed
 - Fix unnecessary refreshes on update preparation failures ([#15261](https://github.com/opensearch-project/OpenSearch/issues/15261))