Inject original chart repo name into Helm chart manifest

After introducing support for multiple Helm chart repositories, user started to ask if they could see somewhere on chart tile rendered in UI what is the original repo of that chart.

Returned entries in index.yaml can be enriched with additional annotations, stating the chart source.

Acceptance Criteria

/api/helm/charts/index.yaml returns entries with annotation whose value is equal to repo name declared in the appropriate HelmChartRepo instance