Add a streamlined `system-reinstall-bootc` or so binary

[cgwalters]

High level goal: Streamline the "install from existing (cloud) instance" flow. Previous work related to this landed in e.g. #919 but we want to make it even easier.

This command will become a frontend for the current podman run <image> bootc install to-existing-root and it will be a little bit more of an "installer". For example it may have a TUI (ncurses style?).

An important goal of this command is to ensure that the admin can SSH into the new instance by default, which may involve automatically propagating things like SSH credentials (which we don't do by default with to-existing-root). It may also make sense to propagate SSH host keys.

Sub-goal: Don't become a full installer

This command should not become a "full" installer, in particular at the moment out of scope are:

• repartitioning
• adding users/groups that don't already exist
• In general most things covered by kickstart/blueprints/etc (timezone, networking, locale...)
• Running arbitrary %post type scripts

Networking and partitioning are the two biggest things and I think people who want that stuff should use externally maintained OS installers (e.g. anaconada) that should be invoking bootc install to-filesystem as part of doing their setup.

[cgwalters]

adding users/groups that don't already exist

This all said, we could probably make it streamlined to run systemd-firstboot against the target root before rebooting, and it looks like that command has things like --copy-locale etc. today too.

[cgwalters]

The initial scope of this is done.