feat: use thread pool

[oraluben]

Use multithread to run IO-bounded jobs (e.g. shell command) concurrently.

Part of #545

TODO:

• [x] pass test with j=1
• [x] fuzz with different j
• [x] cli argument
• [x] doc
• [ ] test
• [ ] test time

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 76.99115% with 26 lines in your changes missing coverage. Please review.

Project coverage is 90.97%. Comparing base (3e4b6b3) to head (1da1db7). Report is 22 commits behind head on main.

Files with missing lines	Patch %	Lines
src/auditwheel/pool.py	47.72%	20 Missing and 3 partials :warning:
src/auditwheel/repair.py	92.30%	2 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #554      +/-   ##
==========================================
- Coverage   92.17%   90.97%   -1.20%     
==========================================
  Files          22       23       +1     
  Lines        1572     1640      +68     
  Branches      286      294       +8     
==========================================
+ Hits         1449     1492      +43     
- Misses         74       95      +21     
- Partials       49       53       +4     

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[oraluben]

Mark as ready-for-review for it passes all tests.

[mayeut]

@oraluben, do you have any timings for this one ? I'd rather not increase the complexity of auditwheel if the improvement is small given maintainer bandwidth.