Add a new distance function to LanternDB index

Currently we only support l2-squared distance function ([sql](https://github.com/lanterndata/lanterndb/blob/main/sql/lanterndb.sql#L19-L22), [c](https://github.com/lanterndata/lanterndb/blob/db801adced391cbb57ecde5c333635dc12f94e62/src/hnsw.c#L226-L231) refs)through usearch.
We should add other distance functions such as l1, l2, hamming distance, cosine distance, etc