Add support for radial search in exact search

[VijayanB]

Description

When threshold value is set, knn plugin will not be creating graph. Hence, when search request is trigged during that time, exact search will return valid results. However, radial search was never included as part of exact search. This will break radial search when threshold is added and radial search is requested. In this commit, new method is introduced to accept min score and return documents that are greater than min score, similar to how radial search is performed by native engines. This search is independent of engine, but, radial search is supported only for FAISS engine out of all native engines.

Related Issues

Part of #1942

Check List

• [x] New functionality includes testing.
• [ ] New functionality has been documented.
• [ ] API changes companion pull request created.
• [x] Commits are signed per the DCO using --signoff.
• [ ] Public documentation issue/PR created.

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[VijayanB]

This depends on https://github.com/opensearch-project/k-NN/pull/2136

[VijayanB]

I am skipping changelog. While merging back to main will update changelog.

[VijayanB]

There doesn't seem to be updates in unit tests, will it be present in another iteration?

Added unit test