Add option to flatten layers in container scan

It would be great to have an option to flatten layers in a container scan. In many common use cases, the lower layers do not matter too much and having a squashed, flat view on a single resulting file system with only "one" layer would be good enough.

The contain-inspector can do this with https://github.com/aboutcode-org/container-inspector/blob/5c654da7c5161060ce57975347526a0e1963313d/src/container_inspector/image.py#L414