Add a new "preview" cluster type

In order to use sylva-units-operator in cli workflow, we need to be able to perform the same validation that we were currently doing in validate_sylva_units called from bootstrap.sh.

For that purpose, instead of relying on postrenderers to pause all flux objects, we rely on a new file that sets all these objects as paused. This file is introduced in sylva-projects/sylva-core!4101 (merged).

Closes: #30 (closed)