Simplified plain json call to opensearch, refactored cluster adapter for os3

[todvora]

/nocl internal code cleanup

Description

This PR simplifies usages around direct request->json response (PlainJsonApi) in opensearch client, integrates the call in the OfficialOpenSearchClient, so we don't need yet another dependency.

This then enables a better testing approach, where we can use a serverless opensearch client where we can pre-register json responses to defined urls. The mocking happens on the transport layer and the original client is used for everything else (see screenshot below)

Motivation and Context

Code cleanup, removed extensive mocking, usages closer to actual code when testing. Tests are now not dependent on exact internal response structure of the client but use json responses returned from the actual opensearch server, increasing compatibility.

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have requested a documentation update.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.