Allow building arbitrary manifests

[supakeen]

image-builder-cli has defaults for many options, it'd be nice if I can build a manifest directly with those defaults.

In combination with #42 or the current image-builder manifest command it allows for rebuilding of images for verification without having to introduce people to a long osbuild command line.

We could have a (hidden) image-builder build-from-manifest command.

[github-actions]

This issue is stale because it had no activity for the past 365 days. Remove the "Stale" label or add a comment, otherwise this issue will be closed in 30 days.

[supakeen]

Not stale.