Add `bootc apply-live`

This is probably mainly draining the logic from rpm-ostree into ostree-ext, then re-using it here.

But...a whole lot of suddenly OS-specific issues come to the fore. For example, should we try to distinguish between "new content" and "changes"?

Start by tracing the apply-live logic in rpm-ostree and the corresponding work in ostree-ext mentioned in the issue. Before implementation, resolve how bootc should distinguish new content from changes and define the OS-specific behavior; the issue does not yet establish concrete completion criteria.

Vom Indexierungsmodell aus dem Issue-Text verfasst.