install: Add `bootc install finalize-filesystem`

[cgwalters]

Today we allow:

• bootc install to-filesystem
• (arbitrary mutation of the target root, i.e. injection of kargs, files in /etc
• reboot into system

For things like #971 but also in general I think we should add:

• bootc install to-filesystem
• (arbitrary mutation of the target root, i.e. injection of kargs, files in /etc
• bootc install finalize-filesystem
• reboot into system

Where finalize-filesystem has to be optional but becomes our "hook" into the process so we can detect or automatically fix things, without having to change each higher level thing (e.g. anaconda, bib, or really arbitrary user code invoking bootc install to-filesystem).

[cgwalters]

The other thing of course we really should do is support something like:

bootc install runc or so to spawn the target root as a container, set up "properly" including how ostree-prepare-root, especially with composefs and honoring etc.transient and such...but unfortunately such a thing would be I think a bit hard to get into e.g. Anaconda right now.