Support user-provided post-renderer for filtering

[briandealwis]

Tracking bug: #6706

#6949 changes our Helm deployer to use Skaffold as a post-renderer. This change will break any users that had configured Helm to use their own post-renderer hooks via deploy.helm.flags. We can have skaffold filter support a --post-renderer flag that then invokes the user-configured post-renderer.

[jearangoo]

is there any progress on this?
i am inserting a post renderer in the flags array [ "--post-renderer=./charts/app/kustomizations/dev/render.sh" ] in deploy.helm.flags.install and deploy.helm.flags.upgrade to add patches, secrets and resources like managed google cloud https certificates, to my chart as they vary between environments, when running skaffold run, skaffold dev and skaffold deploy, but its not working with version 2, maybe this should be linked as well #5387?, thanks.

[Agalin]

This is a blocker for us, we won't be able to migrate to v2 until a way to run post renderers is added.

We're using them to inject secrets, template stuff outside of Skaffold's and Helm's reach and even pipeline results through kustomize at times.