[FEATURE] Put a brief explanation in /run/reboot-required

[jcpunk]

Currently [tracer](https://github.com/FrostyX/tracer/blob/master/tracer/controllers/default.py#L146) puts a message into /run/reboot-required with a brief explanation of why the reboot is required.

Id would be nice if bootc also put a message into /run/reboot-required. This way I could a something like this to /etc/profile.d as a notice to folks:

if [[ -e /run/reboot-required ]]; then
  cat /run/reboot-required
fi

I was thinking something like:

bootc says a reboot is required to load staged image: XYZ

[bshephar]

Does it need to be that specific? Or, could we just do something like: https://github.com/bootc-dev/bootc/pull/1583/files

Note that nothing is calling that function yet, I just saw your issue and thought I would throw something out there for us to discuss in more detail.

We could use a helper function like I've done there and pass it a context of the operation being performed I guess. But I'm just not too sure that additional context adds the value you're after? Compared to just informing users that they need to reboot to finish whatever operation they were doing.

[jcpunk]

Mostly I'd like to know "why" a reboot is pending. In my environment, automated reboots tend to get queued up for once per month, but admins can reboot as needed. Some sort of note as to what is achieved on a reboot can be handy for folks who are open to doing one off reboots.