Allow to customize number of libzim workers

[benoit74]

This PR focus on allowing to customize number of libzim worker ; while probably not the ultimate solution to #38, it at least allows to workaround it until we've understood how to properly apply backpressure (see openzim/python-scraperlib#282)

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 30.30%. Comparing base (327d44d) to head (85aab64).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
scraper/src/maps2zim/processor.py	0.00%	2 Missing ⚠️
scraper/src/maps2zim/entrypoint.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #40      +/-   ##
==========================================
- Coverage   30.32%   30.30%   -0.03%     
==========================================
  Files          11       11              
  Lines        1052     1056       +4     
  Branches      138      139       +1     
==========================================
+ Hits          319      320       +1     
- Misses        722      725       +3     
  Partials       11       11              

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