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
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