feat: add changelog modifier callback to run function

[weichweich]

Description

I added a call back function to manipulate the Changelog before it's written or the context is printed (depending how git-cliff is started).

Motivation and Context

This change is really handy to add custom logic to query information from GitLab etc. I would love to have something similar to plug-ins, but adding this call back is trivial and allows to build custom git-cliff versions that execute use-case specific, custom logic.

How Has This Been Tested?

This should be trivial. What could possibly break!? 😅

Screenshots / Logs (if applicable)

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (no code change)
• Refactor (refactoring production code)
• Other

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have formatted the code with rustfmt.
• I checked the lints with clippy.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 41.15%. Comparing base (82b10ac) to head (8462d82).

Files with missing lines	Patch %	Lines
git-cliff/src/lib.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #922      +/-   ##
==========================================
- Coverage   41.72%   41.15%   -0.57%     
==========================================
  Files          21       21              
  Lines        1678     1699      +21     
==========================================
- Hits          700      699       -1     
- Misses        978     1000      +22     

Flag	Coverage Δ
unit-tests	41.15% <0.00%> (-0.57%)	⬇️

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[orhun]

Hey, really interesting change! I'm down to add it, but maybe add some documentation or examples on how to use this from the API side.
Thanks for the PR!