Add support for applying kustomize to Helm previews

[olensmar]

It's becoming somewhat popular to apply kustomizations to the output of a Helm Chart - see for example https://itnext.io/helm-is-not-enough-you-also-need-kustomize-82bae896816e - we should support this workflow by allowing users to apply kustomizations to the output/preview of a Helm Chart.

My understanding is that the kustomizations can be included/generated by the Helm Chart itself - or managed separately - which would result in slightly different UX workflows:

• Kustomizations generated by a Helm Chart would be available first after a preview, together with their target resources
• Kustomizations managed separately would be available "next to" the Helm Chart - although their target resources would be available only once a preview has been performed!?

Not sure how this is best handled in the UI... perhaps a dedicated flow!?

[chargio]

User feedback: "Is it possible to preview with helm + kustomize ? In a general way, adding the possibility to put custom command would help ! I already have a custom command with argocd plugin and having the same will be awesome"