automatically "ostree-ize" existing containers

A toplevel goal of this project is to support upgrading existing ostree systems in place, so we need ostree support.

However, we also want to further hide ostree and push it into the background. There's multiple interlocking aspects to this.

The biggest one here is to avoid having to carry any "ostree stuff" in the base images by default.

This issue may mostly get fixed by https://github.com/ostreedev/ostree-rs-ext/pull/674 but I'm leaving this open here to track other fixes that may be needed (e.g. https://github.com/containers/bootc/pull/844 was required so far) and also to link to our roadmap.