Unstage via spec

A major goal for this project was to try to get a fully declarative Kubernetes-style spec/status model going.

In practice...the spec basically allows changing the image source, which is fine. However, we need to support things like "unstaging" an update; i.e. to undo the effect of a `bootc upgrade`. Right now it's definitely not ergonomic to do, and we should ensure we have coverage for it.

This overlaps with https://github.com/containers/bootc/issues/383 which is similarly about affecting the bootloader ordering without changing image sources.