feat: expose DatasetPreFilter, PreFilter, and FilterLoader to the pub…

…lic API (#2522)

A [recent
change](36c08d5)
moved these types out of the public API of `lance_index` and into the
private API of `lance`. This broke a customer (full disclosure, us) that
was relying on these types.

These types are well documented and straightforward. I think they were
intended to be part of a public API. However, I could be wrong. Exposing
them allows users to supply their own prefilter implementations when
using `lance_index` utilities directly. I think it would be fair to call
the API experimental if we want to but the prefilter has been stable for
a bit.

rust/lance/src/index.rs

@@ -36,7 +36,7 @@ use vector::ivf::v2::IVFIndex;
 
 pub(crate) mod append;
 pub(crate) mod cache;
-pub(crate) mod prefilter;
+pub mod prefilter;
 pub mod scalar;
 pub mod vector;