Add validation-set support

[Tony Espy]

Please add support to ubuntu-image for validation sets. Ideally this would entail support for a new command-line option that provides a specific validation-set assertion.

It's understood that this feature request has a prerequisite on `snap prepare-image` supporting validation sets (which has been requested from the snapd team for the 22.10 cycle).

[Tony Espy (awe)]

This feature is now supported in by ubuntu-image 3.0 (available from latest/stable) via the `--validation=[ignore|enforce]` command line option for the `snap` command. Unfortunately, I can't tell from git and/or release notes exactly which versions/revisions this landed in, but it should be able in version 3.0+snap12 at minimum.

Please note, that this also requires the validation-set assertion(s) required to be specified in the model assertion per:

https://ubuntu.com/core/docs/reference/assertions/model