[RFC-0012] Add support for ExternalArtifact revision with digest

[stefanprodan]

ExternalArtifact revision can be a semantic version or a digest. This PR adds support for watching ExternalArtifact revision changes and triggering a Helm release upgrade when digests are used.

Part of: fluxcd/flux2#5504
Followup: #1293

This was tested end-to-end using fluxcd/source-watcher#255 and a build of this PR.

---
apiVersion: source.extensions.fluxcd.io/v1beta1
kind: ArtifactGenerator
metadata:
  name: podinfo-gen
spec:
  sources:
    - alias: chart
      kind: OCIRepository
      name: podinfo-chart
    - alias: repo
      kind: GitRepository
      name: podinfo-repo
  artifacts:
    - name: podinfo
      originRevision: "@chart"
      copy:
        - from: "@chart/"
          to: "@artifact/"
          exclude: ["*.md"]
        - from: "@repo/charts/podinfo/values-prod.yaml"
          to: "@artifact/podinfo/values.yaml" # Overwrite the default values inside the chart
---
apiVersion: helm.toolkit.fluxcd.io/v2
kind: HelmRelease
metadata:
  name: podinfo
spec:
  interval: 1h
  releaseName: podinfo
  chartRef:
    kind: ExternalArtifact
    name: podinfo
  install:
    strategy:
      name: RemediateOnFailure
  upgrade:
    strategy:
      name: RetryOnFailure
      retryInterval: 3m

[matheuscscp]

LGTM! 🚀