Make Charts release process automation similar to omnibus-gitlab
Related: #1412 (closed) and gitlab-org/release-tools!645 (merged)
Right now, tag and publish of Charts happen in one uncontrolled pipeline, which means we have a blind spot regarding its release. With #1412 (closed), we are proposing doing at least basic testing (check if the Chart is at least installable) of Charts before they are published to customers. This requires the tag and publish steps of Charts release to be separate, and controlled.
With gitlab-org/release-tools!645 (merged), we are bringing CNG release to be similar to omnibus-gitlab release, which means publishing is a manual job, that will be run by release-tools. We need to implement something similar to Charts too.
Proposal: Make the job that triggers charts.gitlab.io pipeline a manual one, and add code in release-tools to play it as part of publish.