feat: allow user to specify the image to reset to

[jimmykarily]

For installation it's possible to specify which image should be used using:

install:
  source: oci:your_image_here

This works because of the way we parse this key but we don't do anything similar when constructing the ResetSpec.

It's useful to have this option because on public cloud, the system is "installed" by performing an auto-reset. Users can specify the image they want to install by booting any cloud image we push to that provider and setting the image to reset to. This way we don't need to push all possible flavors and such.

Problems with this approach:

• The parent image defines the recovery partition size so if the image specified by the user is bigger, it might not fit.
• We calculate the rest of the sizes based on the "parent" image but this one we can probably fix when we implement this feature.

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[Itxaka]

IIRC, you could override the source during reset by setting it at the image level?

reset:
    system:
      uri: oci:whatever

[jimmykarily]

turns out, this works ^. It's because we do this black magic here: https://github.com/kairos-io/kairos-agent/blob/3412d7a2a2313a8d4ba42a264f11a681f016aaea/pkg/types/v1/common.go#L83-L90

I any case. We should probably find a better way to set the installation/reset/upgrade image so that it's consistent and more intuitive.