Add k8s dashboard backend

Why

See Create a new route and a controller for Kuberne... (#423713 - closed) for more context.

What

• new route for /-/kubernetes/:agent_id
• new controller to handle route
• show action in new controller only returns :ok (for me locally this a blank page if accessed in the browser), but the before actions
  • check the feature flag
  • look up the agent based on the id in the route
  • authorize current_user can read the agent using existing policy
  • set the kas cookie if the authorized and feature flag on

Steps to check locally

Verify with feature flag off

• keep k8s_dashboard feature flag disabled
• identify an id of a Clusters::Agent that's authorized for the user you sign in as
• navigate to https://gdk.test:3443/-/kubernetes/2 (or whatever you local gdk is and id of the agent you found)
• you should see the 404 page
• you should not see | _gitlab_kas | in the cookies in the browser developer tools

Verify with feature flag on

• enable feature flag k8s_dashboard
• navigate to https://gdk.test:3443/-/kubernetes/2 (or whatever you local gdk is and id of the agent you found)
• you should see a blank page
• you should see | _gitlab_kas | in the Response cookies in the browser developer tools