GHA skeleton

[nilsnolde]

so next goal:

• skeleton for GHA, leave placeholders where we need more logic, i.e. some processing scripts
• (for another PR): for processing scripts we'll aim at using python mostly instead of bash, you can get familiar with https://pypi.org/project/pyvalhalla/
• make sure we can process things in parallel if possible
• you can use the S3 machinery for ccache