[python/C++] Release GIL in IntIndexer C++ code

[bkmartinjr]

The Python IntIndexer bindings fail to release the GIL. These are long-running entry points and should release it.

II've benchmarked on some threaded code, and found that this change improves concurrency.

Context: #3345 [sc-57301] [sc-59595]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.76%. Comparing base (67213c9) to head (8ad0260).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3344      +/-   ##
==========================================
+ Coverage   85.66%   85.76%   +0.09%     
==========================================
  Files          57       57              
  Lines        6180     6180              
==========================================
+ Hits         5294     5300       +6     
+ Misses        886      880       -6     

Flag	Coverage Δ
python	85.76% <ø> (+0.09%)	⬆️

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Components	Coverage Δ
python_api	85.76% <ø> (+0.09%)	⬆️
libtiledbsoma	∅ <ø> (∅)

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