[MEVD] [SQL Server] Support approximate/index-based vector search via VECTOR_SEARCH()

[roji]

We currently perform all vector search via VECTOR_DISTANCE(), which performs precise vector search via brute-force (no index). SQL Server 2025 is also introducing VECTOR_SEARCH(), which leverages indexing to perform faster, approximate vector search.

VECTOR_SEARCH() is currently still in preview and is expected to change; once it stabilizes, implement support for creating vector indexes, and if a search is performed over a vector property that has a vector index defined on it, use VECTOR_SEARCH() instaed of VECTOR_DISTANCE().

/cc @yorek