Add filename as valuesKey for valuesFrom of type SylvaUnitsFile

With the introduction of SylvaUnitsFile and Contexts, we'll now sylva-units HelmReleases with this kind of valuesFrom:

  valuesFrom:
  - kind: Secret
    name: sylva-units-file-a96a6e12
  - kind: Secret
    name: sylva-units-file-8a7f6a9e
  - kind: Secret
    name: sylva-units-file-4997f089
  - kind: ConfigMap
    name: sylva-units-values-fbcg2dg8tc
    valuesKey: values
  - kind: Secret
    name: sylva-units-secrets-26k76792g4
    valuesKey: secrets
  - kind: ConfigMap
    name: sylva-units-values-fbcg2dg8tc
    valuesKey: capm3-virt-values
  - kind: ConfigMap
    name: sylva-units-values-fbcg2dg8tc
    valuesKey: capm3-virt-ha-values

It is not obvious to figure-out which source files are being used.

While reviewing !368 (merged), I was considering to ask to add the filename in secret name, but this would have turn in hardly readable names like sylva-units-file-management-values-yaml-a96a6e12 and there were substitutions concerns since object names are constrained in k8s.

On the other hand, it should be fairly straightforward to build a secret with the filename as key, and reference it in the valueFrom:

  valuesFrom:
  - kind: Secret
    name: sylva-units-file-a96a6e12
    valuesKey: management.values.yaml          # <<<
  - kind: Secret
    name: sylva-units-file-8a7f6a9e
    valuesKey: bootstrap.values.yaml           # <<<
  - kind: Secret
    name: sylva-units-file-4997f089
    valuesKey: use-oci-artifacts.values.yaml   # <<<
  - kind: ConfigMap
    name: sylva-units-values-fbcg2dg8tc
    valuesKey: values
  - kind: Secret
    name: sylva-units-secrets-26k76792g4
    valuesKey: secrets
  - kind: ConfigMap
    name: sylva-units-values-fbcg2dg8tc
    valuesKey: capm3-virt-values
  - kind: ConfigMap
    name: sylva-units-values-fbcg2dg8tc
    valuesKey: capm3-virt-ha-values

This way it will be easy to figure-out what the value is referring to.