helm: support Helm chart from GitRepository

[hiddeco]

The current HelmChart spec and its implementation do only support Helm charts from HelmRepository resources. Extending this to also support a chart in the path of a GitRepository would make the resource more suitable for usage by the Helm Operator or its successor.

The HelmChart should have a new field, or a field supporting a type definition with a reference to the GitRepository. In addition, it should be possible to configure the path to the chart relative to the root of the GitRepository artifact.

The chart path should be packaged as a Helm chart using the package action in the action Helm package, or a combination of the chartutil methods, depending on the overhead and availability of package configuration options.