Create a new route and a controller for Kubernetes Dashboard

Proposal

1. Create a new high-level route and a controller for the dashboard. Use /-/kubernetes/. The whole URL will be /-/kubernetes/<agent-id>/, we'll handle the agent-id within the Vue router on the frontend.
2. Use the read_cluster_agent policy in order to keep the authorization consistent across different pages.
3. Add a feature flag with a user actor and hide the new route behind it.
4. Generate the KAS cookie.