Add a preview flag

Enabling the preview.values.yaml preview valuesFile in the valuesFile list enable one to easily test both management and workload clusters.

Currently, a new type has been introduced. It indicates to the present operator to add theis preview.values.yaml file. However, it does not enable to test workload cluster.

The present issue is about to had a new field in the CRD, which is a preview bool flag. If it is true, then, whatever the cluster type, the preview.values.yaml would be added to the valuesFile list of the sylva-units HelmRelease by the present operator