[Helm] Publish helm chart to grafana/helm-charts

[trevorwhitney]

While we are moving the helm chart source back to this repo, we don't want to change where the chart is hosted. As a result we need to incorporate the CI/GitHub Actions from https://github.com/grafana/helm-charts into this repo to publish the new chart to the same place.

• Migrate CI from grafana/helm-charts for testing helm chart changes
• Integrate the github action in the grafana/helm-charts repo (this will publish it, so we will tackle this last)