feat: Add thread names to rayon thread pool

[trueb2]

This adds names to the threads in the Rayon thread pool. They are configurable via the POLARS_THREAD_NAME environment variable and default to polars-{thread_idx}.

Example without and with the change:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.99%. Comparing base (e7be629) to head (c3d7169).
Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15024      +/-   ##
==========================================
+ Coverage   80.98%   80.99%   +0.01%     
==========================================
  Files        1333     1337       +4     
  Lines      173141   173313     +172     
  Branches     2458     2460       +2     
==========================================
+ Hits       140217   140378     +161     
- Misses      32456    32466      +10     
- Partials      468      469       +1     

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[ritchie46]

Wow! I like that!