Add support for hvecs files in `svs.read_vecs`

[mihaic]

SVS generally supports float16 data, including in svs.write_vecs. However, svs.read_vecs lack support for reading such data:

ScalableVectorSearch/bindings/python/src/svs/common.py

Line 111 in a8221ad

raise ValueError('Can only open bvecs, fvecs, and ivecs.')