Implement trigger for downstream Operator pipeline to validate impact of Charts changes

Summary

The Operator supports testing against unreleased charts. We should implement a charts job to trigger a downstream pipeline, to verify downstream impact before merging a charts change.

CNG has a similar downstream Charts trigger CI job that runs a Charts pipeline with the images built as a result of the changes in a CNG MR. This helps us evaluate the impact of changes to the CNG on the Charts project.

gitlab-org/distribution/team-tasks#1331 (closed) was opened to address a similar need for the Operator, which depends on the Charts similar to the way the Charts depends on the CNG.

Acceptance criteria

• Trigger branch created
• Trigger branch is updated by automation
• A charts MR pipeline has an manual job, to trigger a Operator pipeline