feat: cache btree sub-index pages

[westonpace]

The btree currently only caches the the lookup. This means there will always need to be at least one IOP per search 4Ki rows that match the filter to load the sub-index (flat) data.

This PR adds a cache for the data pages as well.

The cache defaults to 512MiB and is technically configurable via LANCE_BTREE_CACHE_SIZE but this is just a stop-gap. We should ideally have a single size limit for all scalar indices (or, even better, a single size limit for ALL indices). However, this is a bit tricky. I don't think moka caches can share capacity across several caches and using a single moka cache for all places we need cache is tricky because moka caches are typed (in some places we use Arc<dyn Any> which might be a possible solution though we still need to deal with the fact that we key the different caches by u32, String, and object_store::Path).

We can resolve this technical debt in #3136

[westonpace]

See https://github.com/lancedb/lance/actions/runs/12517440943 for a test of the newly introduced CI benchmark

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 82.22222% with 8 lines in your changes missing coverage. Please review.

Project coverage is 79.03%. Comparing base (3a47444) to head (be88580).

Files with missing lines	Patch %	Lines
rust/lance-index/src/scalar/btree.rs	82.22%	4 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3309      +/-   ##
==========================================
+ Coverage   79.02%   79.03%   +0.01%     
==========================================
  Files         246      246              
  Lines       87589    87628      +39     
  Branches    87589    87628      +39     
==========================================
+ Hits        69217    69259      +42     
+ Misses      15506    15502       -4     
- Partials     2866     2867       +1     

Flag	Coverage Δ
unittests	79.03% <82.22%> (+0.01%)	⬆️

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