Add a trigger job for charts

What does this MR do?

Adds a manual trigger for running a chart pipeline with the built images.

Relies on a few charts MRs to be merged first:

• change image versions from ci variables - gitlab-org/charts/gitlab!2195 (merged)
• have a mirror branch for trigger pipelines - gitlab-org/charts/gitlab!2198 (merged)
• change environment name/slug from ci variables - gitlab-org/charts/gitlab!2200 (merged)

Related issues

Part of gitlab-org/charts/gitlab#2824 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated