Implement trigger for downstream Operator pipeline to validate impact of Charts changes
## Summary
[The Operator supports testing against unreleased charts](https://gitlab.com/gitlab-org/cloud-native/gitlab-operator/-/issues/1578). 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](https://gitlab.com/gitlab-org/build/CNG/-/blob/c4ed59a3386c077b09dcacf9b0364c0e65030914/.gitlab/ci/images.gitlab-ci.yml#L17-49)
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.
https://gitlab.com/gitlab-org/distribution/team-tasks/-/issues/1331 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
issue