Enhanced OpenSearchVectorStore - Squashed

[JM-Lab]

These changes enable support for both Exact k-NN and Approximate k-NN searches in OpenSearch. The code was updated and refactored to include this new functionality, as it previously only supported Exact k-NN. This work is based on the squashed changes from Enhance OpenSearchVectorStore #987.

• Enhanced OpenSearchVectorStore to support both exact and approximate k-NN search methods.
• Updated integration tests to cover common similarity metrics (cosinesimil, l2, innerproduct) for both exact and approximate k-NN.
• Made necessary updates to the auto-configuration to accommodate the new support for approximate k-NN.
• Updated the OpenSearch Vectorstore documentation to reflect these changes.

[JM-Lab]

@tzolov @markpollack
Hello, I was wondering if there's been any progress on reviewing?