Initial helm support

Relates to gitlab-org/gitlab#370523 (closed).

Closes gitlab-org/gitlab#371019 (closed).

Known issues/limitations/quirks:

• The only way to provide values.yaml is to commit it together with the chart.
• The only way to provide the chart is to commit it to a repository agent is authorized to access.
• Chart is re-installed ("upgrade") every 5 minutes, creating a "release". Number of releases is limited to 1 (by default), but is configurable.
• No way to use post-processing capabilities.
• Only configurable options are max history (defaults to 1) for releases and namespace (defaults to "default"). Everything else should be added in follow up MRs.
• ...