feat: Use remote OCI image references as rootfs element

[nderjung]

Prerequisite checklist

• [x] Read the contribution guidelines regarding submitting new changes to the project;
• [x] Tested your changes against relevant architectures and platforms;
• [x] Ran make fmt on your commit series before opening this PR;
• [ ] Updated relevant documentation.

Description of changes

This PR makes it possible to specify a remote OCI image as a rootfs. This prevents having to use BuildKit in order to retrieve the rootfs, which has become a common operation.

[nderjung]

Pipeline is failing due to a different cause, fixed in https://github.com/unikraft/kraftkit/pull/1631.

[craciunoiuc]

@nderjung you'll have to rebase again :D