Allow `ExternalArtifact` as a source in `ArtifactGenerator`

[stefanprodan]

We should add the ExternalArtifact as a source in ArtifactGenerator to integrate with 3rd-party source controllers as per RFC-0012.

xref: fluxcd/flux2#5504

[matheuscscp]

This feature will require the introduction of a .spec.dependsOn field similar to Kustomization and HelmRelease.