[DuckDB] Expose vector search capability via duckdb extension

[eddyxu]

Problem Statement

Lance rust dataset has the capability of querying over vector space, via Scannner::nearest(..), we should expose this to DuckDB via its extension mechanism.
For example, a potential implementation could be a duckdb table function lance_nearest

SELECT id, col 
FROM lance_nearest("table", "vector_column", "query", top_k)
WHERE city = 'SF' and day = "02-03-2023"

[changhiskhan]

Ideally duckdb extension would let us modify the query plan so if we see ORDER BY nearest(...) LIMIT 10 then this is pushed down as ANN search