Implement --skip-prefixes parameter

[iamasmith]

• Allows specific image prefixes to be skipped from digest resolution
• Implemented for webhook and kpt
• kpt support also includes environment variable SKIP_PREFIXES bound to the same parameter
• refactors kubeconfig parsing of multi value strings into util.StringArray for reuse

[iamasmith]

First try, I tried to keep with the theme but there's always something that could be better so let me know if you would like to use it but want changes please.

[costinm]

Suggestion: can we also add a label ? I.e. if the pod label matches, skip ?

[iamasmith]

@costinm I think this is a separate use case. Sounds great but it's probably a separate ask on Issues to be considered with use cases. To clarify, this is about skipping selective containers within a pod, not the pod overall.
Editing again on this.. are you suggesting making this specific feature dependent upon the label rather than capable as an overall fix for this specic issue? If so as a minimum it's probably an ask for the label check to be on the namespace since if somebody required this broadly it would be quite onerious.

[halvards]

Thanks @iamasmith for the PR!

The changes look good, and I also really appreciate that you've added tests and updated the documentation.

I'll merge and push out a release.

@costinm, your request sounds like another feature that could be added. Feel free to raise another issue to track it.