Refine valuesFrom API to make it more generic and flexible.

Current API used to define the layer of values files included in bootstrap or preview is not very flexible since these files are hardcoded here.

It is hard to define new scenarios, and/or to extend existing ones. For example, some users may need to add specific values in the context of bootstrap, this is currently not possible.

Additionally, in order to implement #45 (closed), we'll have to define these files as valuesFrom instead of valuesFiles.

The proposed solution would consist in adding a new SylvaUnitsFile type to valuesFrom to allow user to refer to files located in sylvaUnits repository, and add an optional context field to these valuesFrom to only include these files in a specific context (bootstrap, preview...)

spec:
  contexts:                                     # Determine which context are included
    bootstrap: true                             # Would be removed after pivot
    preview: true
  sylvaUnitsSource:
    type: git
    url: SYLVA_CORE_REPO
    commit: CURRENT_COMMIT
  valuesFrom:
    - type: SylvaUnitsFile
      valuesPath: values.yaml
    - type: SylvaUnitsFile
      valuesPath: management.values.yaml
    - type: SylvaUnitsFile
      valuesPath: bootstrap.values.yaml
      contexts: bootstrap                        # Indicate that this value should only be included in bootstrap context
    - type: OCIRepository
      name: third-party-bootstrap
      valuesPath: bootstrap.yaml
      context: bootstrap                         # Additional files could be injected from external repos or configmaps in bootstrap context
    - type: SylvaUnitsFile
      valuesPath: preview.values.yaml
      context: preview                           # Same for preview
    - type: ConfigMap
      name: sylva-units-values
      valuesKey: values
    - type: Secret
      name: sylva-units-secrets
      valuesKey: secrets

This would be much more flexible, and would ease the definition of additional behaviors (to preview workload cluster helmrelease for example)