[FEATURE] API Specification updates for OpenSearch Reader/Writer Separation

[prudhvigodithi]

Is your feature request related to a problem?

Update the API spec related to Reader/Writer Separation flow.

What solution would you like?

• Update the spec with search nodes.
• Have a story for search replicas.
• Have a story and tests for scale to zero (search_only mode)
• More details which can be added to the spec [META] Reader/Writer Separation OpenSearch#15306 (comment)

[prudhvigodithi]

Here is the initial PR #883 to just ensure the API spec is compatible with 3.0.0-beta1. I have created this issue so that we can have end to end API spec updates for OpenSearch Reader/Writer Separation.
Thanks
@mch2 @getsaurabh02 @vinaykpud