Add GitLab config for KAS Kubernetes API address

Problem to solve

The CI/CD tunnel URL is hardcoded #{external_url}/k8s-proxy, but this is only true for the GitLab Helm chart. Other GitLab setups, such as GDK and Omnibus, would typically not have the tunnel on the same port as the agentk endpoint.

Proposal

Add a GitLab configuration option

gitlab_kas:
  external_k8s_proxy_url: <url>

More details

Inside KAS, the Kubernetes API runs on a separate port. The Helm chart is a special case because the ingress resource merges the gRPC endpoint and the Kubernetes API in a single port.