Publish helm chart as OCI artifacts

[ChristianCiach]

OCI artifacts are the modern way to publish Helm Charts. Most well known projects already publish their charts this way (in addition to the traditional HTTP repos).

Most projects hosted on Github just publish the Charts to ghcr.io directly from their Github workflow, and I propose Keel should do the same.

Example:

• https://github.com/stakater/Reloader/pkgs/container/charts%2Freloader
• https://github.com/stakater/Reloader/blob/6b2cc9446843cb039fc795b08ab32a8fdd618e88/.github/workflows/push-helm-chart.yaml#L84