Add option to create summary file for OSTree commits

[pk-robom]

I think it would be nice to have an option for OSBuild to automatically create a summary file for OSTree commits, as one would do with:

ostree --repo=repo summary -u

[teg]

Sounds like the kind of thing we could return in the result from osbuild.

Could you describe a bit how you imagine using this?

[pk-robom]

When downloading an, e,g., iot-commit with composer-cli compose image, it would be nice if a summary file would be created before the .tar archive is created, just as a convenience so that one does not has to do it himself.
Maybe with a cli option (e.g., --summary).