sylva-units: introduce globally versioned Kustomization dependencies

This MR addresses #1200 (closed) as a follow-up to !2097 (merged):

• it introduces a root-dependency unit that:
  • is in the depends_on of all units
  • becomes ready only after all Kustomizations resource have been updated and processed once by Flux Kustomization controller
  • has a dynamic name prefixed by the sylva-units Helm release revision, to ensure that on an upgrade all Kustomization reconciliations will follow the dependencies chain
• (it also adds unit_definition_defaults as a means to easily add something to all unit definitions)
• reverts the parts of !2097 (merged) that are not necessary anymore

(💡 best reviewed one commit at a time)

Beyond some simplification of the code introduced in !2097 (merged), this MR enhances sylva-units by introducing a generic mechanism that ensures that we control the order in which units updates are applied during an upgrade of a sylva-units Helm release.