status: add diff for cached update

[cgwalters]

Moving this from #472 - we should add a serialization of the diff for cached updates to status.

We should also print it in the human readable version.

[antheas]

Since I have now built the updater, here are some missing pieces

bootc status --json covers bootc update --check but only that. There is no bootc switch --check, so when performing rebases we are blind to the next version. Should there be a bootc switch that only changes the remote? Seems it would be convoluted.

If the next version cache has access to the image manifest, annotations should be included to allow for higher level tools to act on them without requiring pulling the image manifest out-of-band, which might also inflate pull counts.

The annotations could be used for showing important image information to the user, or, e.g., provide automatic rebasing in case of image deprecation.