Implement Garden Linux "Features" for OSTree

[fwilhe]

What would you like to be added: The gardenlinux/gardenlinux repo has a lot of features which are not reflected in this repo. Some of the features just add packages and can be built into the base image, but for some features it might make also sense to package them as a system extension or by other means.

Why is this needed:
It is not sustainable to make too many image variants (like with curl or without curl), so having the _curl image as a system extension that can be easily enabled would make more sense compared to baking it into the image itself. This approach won't work for all features, so they need to be categorised and analysed.