Add proper typing @overload to zimscraperlib.image.optimize_xxx methods

[benoit74]

Type hints could be more precise thanks to @overload python feature (see https://typing.python.org/en/latest/spec/overload.html)

[codecov]

Codecov Report

❌ Patch coverage is 53.84615% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.63%. Comparing base (26e3ab1) to head (1893e14).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/zimscraperlib/image/optimization.py	53.84%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #273      +/-   ##
==========================================
- Coverage   99.87%   99.63%   -0.25%     
==========================================
  Files          41       41              
  Lines        2480     2493      +13     
  Branches      340      346       +6     
==========================================
+ Hits         2477     2484       +7     
- Misses          1        7       +6     
  Partials        2        2              

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[rgaudin]

Lacks tests ; otherwise LGTM

[benoit74]

Sorry, I forgot to fix PR number in CHANGELOG, hence the force push

[benoit74]

I don't think it lacks tests, this is just a "bug" in coverage report. These methods are "fictive" and never really executed