Write workflow to maintain release PR

[martijnarts]

In my initial idea of the release flow, we'll have a workflow run on main that maintains a Git branch and Github PR that would release any draft changes in main. We need to:

• Write the workflow
• Write tests for it
• Document it
• Extend the script created in Finish pr-changelog.yml #3 to add any potential additionally needed secrets

The workflow should:

1. Check out (and optionally create) the release-draft branch
2. Clear any .changelog/vX.Y.Z.json files made in this branch
3. Regenerate the CHANGELOG.md and .changelog/vX.Y.Z.json file from the .changelog/draft/ directory
4. Commit and push the changes
5. If necessary, create a PR on Github