[Extensions] OpenSearch requests should know the identity of the caller

[peternied]

Is your feature request related to a problem? Please describe.
OpenSearch extensions need to know the identity of the caller and existing architecture requires depending on the security plugin to get identity information. This creates an additional complication dependency or a source of runtime failure. Neither are easy to manage when compared to having these concepts inside of OpenSearch.

Describe the solution you'd like
Extensions should have a dependable identity model and objects from OpenSearch. There should be identity service, objects, and APIs that are codify in this codebase. This would remove the need for additional dependencies like common-utils for these scenarios.

Describe alternatives you've considered
Leave the existing model unchanged, this puts the burden of managing the interface on external repositories

Additional context
Within the extensions features there has been discussion on how identity should be handled, opensearch-project/opensearch-sdk-java#14.

[peternied]

To support user identity, there needs to be an authentication system that can check with an identity store (local/remote). The identity store would resolve the user identity to a reference that can be passed around within OpenSearch to the request handlers.