feat: `dry-run` flag for kairos-agent upgrade

[jasperdekeuk]

Is your feature request related to a problem? Please describe.

I was mainly looking for a solution to see what image kairos-agent would upgrade to, also when using a private registry, to see that the device can access the registry, before actually doing an upgrade of the system. I was using the list-releases for this, but might not be ideal. -> see: #3814

It made me wonder if a dry-run flag for the kairos-agent would make any sense for UX?

Describe the solution you'd like

Basically it should run everything that the kairos-agent upgrade does, but it should not actually do an upgrade. The output should be a lot of interesting meta info, like what image will be updated to (so you also see that the source of the cloud-config is used etc.) that e.g. the private registry is reachable, and potential other interesting info that could come from things that are usually done with an upgrade.

Describe alternatives you've considered

Additional context

This could potentially replace the list-releases as it could e.g. also output things like "no new version available" or "newer version available, will update to version x.y.z"

[jasperdekeuk]

if e.g. a source is changed, it could also output something like "source changed from ... to ..."

[Itxaka]

umm this is more complex that it appears as there is some things that are calculated during runtime IIRC. Things like whre to store the transition image, the size of the source/image, etc..

We could maybe try something like a summary instead? Which would try to resolve most of the possible things like location to store the transition image, source, size and so on and report that before the upgrade for a manual sanity check. Not sure if that would fit what you are looking for @jasperdekeuk ?