feat: Move test formatter from .github/workflows/format_diff.py to python-gardenlinux-lib

[Leon-hk]

What this PR does / why we need it:

• For the still unmerged reproducibility checker, I've written a format_diff.py script which generates a readme report for the workflow run
• This script manually parsed the flavors directory and created feature trees. But this can also be way easier and more reliable done using the python-gardenlinux-lib
• This PR ports the readme generator to the python-gardenlinux-lib in order to clean up the workflows directory, to use features of the library and to be able to test the output with simple unit tests

[codecov]

Codecov Report

❌ Patch coverage is 86.77249% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.14%. Comparing base (9bc3c1f) to head (7130e57).

Files with missing lines	Patch %	Lines
.../gardenlinux/features/difference_formatter_main.py	0.00%	23 Missing ⚠️
src/gardenlinux/features/difference_formatter.py	98.79%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #294      +/-   ##
==========================================
- Coverage   91.54%   91.14%   -0.40%     
==========================================
  Files          42       44       +2     
  Lines        2070     2259     +189     
==========================================
+ Hits         1895     2059     +164     
- Misses        175      200      +25     

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