Handle the upgrade of flux-system

In order to be able to use flux features as soon as we integrate a new flux version, we have to upgrade flux prior to upgrade the sylva-units helmRelease, otherwise we'll face issue like sylva-projects/sylva-core#2375 (closed) again.

For that purpose, sylva-units-operator should patch the existing flux-system kutomization to use the new gitrepo and wait for its reconciliation prior to upgrade sylva-units HelmRelease.

Note that for that purpose, we'll have to configure any new configurable feature as a kustomize component, tu ensure that it won

version N kustomization (patched by the operator)

spec:
  components:
  - ../components/extra-ca
  path: ./kustomize-units/flux-system/base
  postBuild:
    substitute:
      SOURCE_CONCURRENT: "2"
      SOURCE_FEATURES_GATES: ""        # <<< parameters set by sylva-units HelmRelease version N
  sourceRef:
    kind: GitRepository
    name: sylva-core-N+1               # <<< using the N+1 kustomization

version N+1 kustomization (updated by sylva-units HelmRelease version N+1)

spec:
  components:
  - ../components/new-kustomize-feature      # <<< overlay needing the new parameter added later on as a component
  path: ./kustomize-units/flux-system/base
  postBuild:
    substitute:
      SOURCE_CONCURRENT: "2"
      SOURCE_FEATURES_GATES: ""
      KUSTOMIZE_NEW_FEATURE: foo             # <<< new parameter set by sylva-units HelmRelease version N+1
  sourceRef:
    kind: GitRepository
    name: sylva-core-N+1

Here is an overview of the proposed upgrade workflow: