Allow the restriction of Kubernetes agent access to protected branches

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What does this MR do and why?

This MR allows the restriction of Kubernetes agent access to protected branches, see #467936 (closed).

🛠 with ❤ at Siemens

MR acceptance checklist

MR Checklist ( @nwittstruck)

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the javascript style guides
• Conforms to the database guides

Screenshots or screen recordings

kas-demo

How to set up and validate locally

1. Prerequisites:

• Your GDK must be configured to run GitLab Runner and GitLab Agent Server (KAS)
• Connect your Kubernetes cluster by installing agentk to the cluster.
• Enable the feature flag with rails c:

Feature.enable(:kubernetes_agent_protected_branches)

2. You will need to include the changes of this MR of the GitLab Agent for Kubernetes. Your GDK should be set up to run the agent from source:

gitlab_k8s_agent:
  enabled: true
  run_from_source: true

3. Create a repository that contains the following file .gitlab/agents/k8s-kas-dev/config.yaml configuration to a repository:

ci_access:
  projects:
    - id: dev/gdk-kas-dev # This must be a valid project id in your GDK setup
      protected_branches_only: true # Toggle this flag to allow access only from protected branches

4. You can now create a .gitlab-ci.yml file in (un-)protected branches. Based on the setting of ci_access, you should either see a list of contexts or not.

deploy:
  image:
    name: bitnami/kubectl:latest
    entrypoint: ['']
  script:
    - echo "Listing pods from protected branch"
    - kubectl config get-contexts
    - kubectl config use-context dev/gdk-kas-dev:k8s-kas-dev # use your local context
    - kubectl get pods --all-namespaces