ci: Add workflow to automatically add external contributors to CHANGELOG.md

[mydea]

This adds a new GHA job that, if a PR is created by an external contributor, it will open a PR against that branch that adds the contributor to the changelog, so we do not forget about this.

It will open a PR like this:

#12435

which we have to manually merge/review, so nothing "bad" can happen.

[mydea]

do not run this on each update, as it is kind of wasteful (it should not actually error, just unnecessary computation)

[Lms24]

Looking at the example PR, shouldn't the workflow have appended the new contributor to the already mentioned one? We could also just keep a list of contributors in the ##Unreleased section to simplify the appending process

[Lms24]

Regardless, I'm fine with either way, as long as we don't by default overwrite already mentioned contributors 😅

[mydea]

Looking at the example PR, shouldn't the workflow have appended the new contributor to the already mentioned one? We could also just keep a list of contributors in the ##Unreleased section to simplify the appending process

It should! I forced it to only add a single entry to test how it looks (I manually removed it in the changelog)

[mydea]

We could also just keep a list of contributors in the ##Unreleased section to simplify the appending process

I added some logic to handle this, hopefully. It's regex-y, but I tested the different scenarios and it should work to append stuff correctly if it exsits!