[KOTS]: add ability to use custom workspace image stored in different private registry

[mrsimonemms]

Is your feature request related to a problem? Please describe

Whilst working with @nandajavarma on custom workspace images, we discovered that you can use a private image but that it MUST be in the same registry as the workspace images are being stored in.

Describe the behaviour you'd like

• Add "advanced" option in KOTS
• Add option for "default workspace image" (defaulting to gitpod/workspace-full:latest)
• Add option for registry credentials - these will then be appended to the registry secret, as per [kots]: allow multiple docker pull secrets #10685
• The registry credentials will be validated by the preflight check

Describe alternatives you've considered

Insist upon either:

• public image
• private image stored in same registry as where workspace images are stored

Additional context

#8550 seems to be how it's done for SaaS, but this doesn't work for Self-Hosted. Also, it's likely to be desirable for this to be configured by the instance owner.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.