[toolforge_deploy_mr.py] support deploy of MRs from external contributors
- add public registry to helmfile.yaml and values file when neccessary
Bug: T394595
Depends-on: toolforge-deploy!958 (merged)
Depends-on: repos/cloud/cicd/gitlab-ci!61 (merged)
Signed-off-by: raymond-ndibe rndibe@wikimedia.org