Use a kustomization to deploy external-secret-operator

Summary

Since we intent to pre-upgrade external-secret-operator as we do for flux and sylva-units operator, we should use a kutomization instead of an helmRelease, otherwise the pre-upgrade only results in upgrading the dummy kustomization...

At that point we'll be able to add the sylva-units.sylva-project.org/release-pre-upgrade label on that kustomization, and remove the dependency on cluster-machines-ready unit for the reason explained in !6436 (merged)

We'll also probably have to add a dependency between sylva-units-operator and external-secret operator as soon as we'll add the capability to read valuesFrom externalsecrets.