Add support for update --apply

[cgwalters]

Today we have e.g. rpm-ostree upgrade --reboot. bootc update does not have an equivalent verb.

In this proposal we add update --apply which would today by default reboot. In the future ostree should gain support for systemd soft-reboot.

A bit longer term than that, we'll want to have rich tooling for admins/tooling more directly controlling the flow here.

First, an important detail is that I think we should default to locking finalization so that a reboot for other reasons doesn't apply the update - this ensures predictability for the sysadmin. This would be exposed as a new locked: bool in the CRD for staged deployments that could be changed to false if the admin/upgrade tooling wanted a reboot to also apply by default.

Also, what I think would be helpful here is to add some sort of "diff" to the CRD as well that says what the "apply strategy" would be (reboot, soft reboot or live apply).