feat(changelog): changelog_message_build_hook can now generate multiple changelog entries from a single commit

[noirbizarre]

Description

This PR allows to changelog_message_builder_hook to generate multiple changelog entries from a single commit (sequel to #1001).

This PR includes #1002 as it touches the same part (so first commit will automatically be removed as soon as #1002 is merged)

Checklist

• Add test cases to all the changes you introduce
• Run ./scripts/format and ./scripts/test locally to ensure this change passes linter check and test
• Test the changes on the local machine manually
• Update the documentation for the changes

Expected behavior

If you return an Iterable of parsed messages (aka. dicts) instead of a single one, you can add multiple changelog entries for the same commit.

Steps to Test This Pull Request

1. Create a custom changelog_message_build_hook
2. Return an iterable of entries
3. Generate changelog without failure
4. Check that all entries are in the changelog

Additional context

Follow #1001
Requires #1002 which is included

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.44%. Comparing base (120d514) to head (1c207df).
Report is 199 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1003      +/-   ##
==========================================
+ Coverage   97.33%   97.44%   +0.11%     
==========================================
  Files          42       55      +13     
  Lines        2104     2391     +287     
==========================================
+ Hits         2048     2330     +282     
- Misses         56       61       +5     

Flag	Coverage Δ
unittests	97.44% <100.00%> (+0.11%)	⬆️

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