[FEATURE] Fill out missing specs of this plugin in the API Spec Repo

[nhtruong]

Many endpoints of this plugin have yet to be described in the OpenSearch API Spec repo. Without proper specs, we won't be able to generate clients that can serve these endpoints.

For more details, please check the Developers Guide and the related issue on the spec repo:
opensearch-project/opensearch-api-specification#227

[dblock]

[Catch All Triage - Attendees 1, 2, 3, 4, 5]