Implemented filtering on the ISM eplain API (opensearch-project#998)

* Implemented filtering on the ISM eplain API

Signed-off-by: Joshua Au <[email protected]>

* Fixed tests for ExplainRequest

Signed-off-by: Joshua Au <[email protected]>

* Added filtering on query and metadata map

Signed-off-by: Joshua Au <[email protected]>

* Filtered on indexNames in metadata

Signed-off-by: Joshua Au <[email protected]>

* Fixed github workflow check errors

Signed-off-by: Joshua Au <[email protected]>

* Removed debugging comments

Signed-off-by: Joshua Au <[email protected]>

* Updated code styling to make more clear

Signed-off-by: Joshua Au <[email protected]>

* Refactored code to match suggestions

Signed-off-by: Joshua Au <[email protected]>

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Signed-off-by: Joshua Au <[email protected]>
Co-authored-by: bowenlan-amzn <[email protected]>