First take at ConventionalCommitsAppenderCz

[stdedos]

Description

Checklist

• Add test cases to all the changes you introduce
• Run ./script/format and ./script/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

I would like to have ConventionalCommitsCz, but also be able to extend it locally, as per project requirements/wishes - also to fit local workflows (i.e. use fixup!, squash!, or throwaway commits without the extra pain of disabling the local commit hooks)

As indicated by the PR, this is just a RFC: I would like to get the project's insight on how this could be achieved, since these changes touch on internal committizen parts that you might not necessarily want changed.
Also, I failed to find a tutorial on how to create my own plugin.
Finally, I've never used poetry before; and therefore everything is broken/unknown to me.

Steps to Test This Pull Request

Additional context

[woile]

Could this be started as a separate project? Like one of the ones listed here: https://commitizen-tools.github.io/commitizen/third-party-commitizen/