add fluxcd labels post processor

[kvaps]

This PR introduces a new fluxcd-kustomize.sh script that can be used as post-processor for helm for adding a common fluxcd labels.

This is very useful for make diff, so it will not include diff between these labels anymore

Also for debugging specific kustomize cases, eg:

• https://github.com/fluxcd/helm-controller/issues/283
• https://github.com/fluxcd/flux2/issues/4368

[kvaps]

it seems this is a part of one huge task https://github.com/aenix-io/cozystack/issues/187