Allow `HelmChart` as a source in `ArtifactGenerator`

[stefanprodan]

Currently in ArtifactGenerator, we support vendoring charts from container registries, where users can replace the values.yaml or merge it with values files coming from Git or S3 buckets.

In the case of charts coming from legacy HTTP/S repo where source-controller generates a HelmChart object, the only way to modify the values is using a Flux Kustomization that applies the values from Git as a ConfigMap, then in HelmRelease you would use valuesFrom. This procedure adds a lot of overhead, so we can offer a better alternative by allowing HelmChart as a source in ArtifactGenerator, that will enable values merge in the same way as with charts coming from OCIRepository.