Add `--stateroot` to `bootc switch`

ostree supports setting a different `--stateroot` so different distros can be installed into the same ostree. Bootc (when it was purely ostree based) was technically capable of doing the same, but it has no options for doing so.

https://github.com/bootc-dev/bootc/pull/617 started to implement this, but then split and morphed into adding `--stateroot` to `bootc install`. Unfortunately `bootc install` wipes the `/boot` or fails entirely, which is kind of it's intended purpose, so it doesn't actually match the behavior.