[BUG] KnnQuery radial search spec inconsistency

[alex-keeler]

What is the bug?

The KnnQuery spec (located at schemas/_common.yaml#/components/schemas/KnnQuery) is inconsistent with the current implementation of the knn query, specifically in regards to radial search. It specifies a similarity parameter, whereas the implementation accepts a min_score or max_distance parameter to achieve the same end.

Do you have any additional context?

Radial search docs: https://opensearch.org/docs/latest/search-plugins/knn/radial-search-knn/
Original issue: opensearch-project/k-NN#814

[dblock]

Thanks for opening this @alex-keeler. Want to try and fix it? We have a nice test framework.