KOTS: prerender yaml & validate patch files, prior to proceeding with deploy

[kylos101]

Is your feature request related to a problem? Please describe

Customer patch files do not get validated before we deploy. This can be painful during outage windows, where they're trying to proceed with an upgrade, have a malformed file, but don't necessarily know why the upgrade is failing.

Describe the behaviour you'd like

A pre-flight check should fail if the patch file causes rendered yaml to fail validation. This would ideally prevent the upgrade from continuing and clearly show the administrator what failed.

Describe alternatives you've considered

n/a

Additional context

[kylos101]

@lucasvaltl @corneliusludmann a customer ran into this issue recently when doing an update of a self-hosted Gitpod installation.

cc: @atduarte @aledbf

[corneliusludmann]

Thanks, @kylos101 for raising this feature request. I'll close this as a duplicate of #12015.

We couldn't implement this easily in the past. However, the refactoring @mrsimonemms did recently enables this. We will have this in the next release.