Retrieve vectors 5x faster with docvalue_fields in OpenSearch

OpenSearch 3.7 introduces the ability to retrieve vectors via `docvaluefields` rather than the traditional `source` path, allowing for direct reading from the columnar store. This optimization eliminates expensive data reconstruction processes, resulting in up to 5.5x faster end-to-end latency and significantly smaller response payloads when using Base64 format. Importantly, this feature works on all existing indexes without requiring any reindexing.