feat: make the # of CPU threads configurable and document cpu/memory patterns

[westonpace]

This gets rid of all usage of num_cpus (except in one spot to determine the default # of CPU threads) and instead uses ObjectStore::io_parallelism or get_num_compute_intensive_cpus. Admittedly, many of our operations do not split out I/O and compute. For example:

async move {
  let batch = read_batch(...).await?;
  let transformed = transform_batch(batch);
}

However, we can just pick one and clean these up as we go. For now, I made a best effort guess (most places where we were applying a multiplier to num_cpus I use the I/O count. Other places I use the compute count.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.25287% with 5 lines in your changes missing coverage. Please review.

Project coverage is 79.30%. Comparing base (eb87bfa) to head (57d076d).
Report is 1 commits behind head on main.

Files	Patch %	Lines
rust/lance-core/src/utils/tokio.rs	50.00%	1 Missing and 1 partial ⚠️
rust/lance-io/src/object_store.rs	71.42%	1 Missing and 1 partial ⚠️
rust/lance/src/index/vector/ivf/v2.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2773      +/-   ##
==========================================
+ Coverage   79.26%   79.30%   +0.03%     
==========================================
  Files         227      227              
  Lines       68245    68283      +38     
  Branches    68245    68283      +38     
==========================================
+ Hits        54096    54153      +57     
+ Misses      11023    11002      -21     
- Partials     3126     3128       +2     

Flag	Coverage Δ
unittests	79.30% <94.25%> (+0.03%)	⬆️

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[eddyxu]

can io_parallalism return usize so dont need cast all of them?

[westonpace]

Good idea. Done.