Implement CLI command to be used as kube config credentials plugin

Problem to solve

As a follow-up on Provide a user-based kubeconfig using the GitLa... (&6038), we would like to integrate the local cluster connections with glab.

As a Developer, in order to access the clusters I'm authorized to reach

• I want to easily list the GitLab-cluster connections I can access
• I want to easily update my local kubeconfig with a selected connection
  • I want to create a separate personal access token for cluster access

Proposal

Implement a Kubernetes credentials plugin which can be used in a user kube config entry to retrieve a k8s_proxy-scoped PAT to access an agent.

The command may be implemented as: glab cluster agent get-token --agent=<agent-id> and configured via glab cluster agent update-kubeconfig --agent=<agent-id> to something like this:

apiVersion: v1
kind: Config
clusters:
- cluster:
    server: https://kas.gitlab.com/k8s-proxy
  name: gitlab
contexts:
- context:
    cluster: gitlab
    user: gitlab-user
  name: gitlab-agent
current-context: gitlab-agent
preferences: {}
users:
- name: gitlab-user
  user:
    exec:
      apiVersion: "client.authentication.k8s.io/v1"
      command: "glab"
      args:
        - cluster
        - agent
        - get-token
        - --agent
        - 1234
      interactiveMode: IfAvailable

Links / references