fix(cluster-agent): construct kubeconfig authinfo name with agent id

Description

This change set constructs the auth info in the generated kube config entries for the glab cluster agent update-kubeconfig command with the agent id instead of the username. This avoids conflicts where a glab user can switch their account but want to reuse the same agent or simply using a different token. More importantly though it generates the same auth info for two different agents which can cause problems.

The changed command generates a kubeconfig along the lines of:

apiVersion: v1
clusters:
- cluster:
    server: https://kas.gitlab.com/k8s-proxy
  name: gitlab_com
contexts:
- context:
    cluster: gitlab_com
    user: gitlab_com-1096606
  name: gitlab_com-timofurrer_playground-dev
- context:
    cluster: gitlab_com
    user: gitlab_com-1100000
  name: gitlab_com-timofurrer_playground-foobar
current-context: gitlab_com-timofurrer_playground-foobar
kind: Config
preferences: {}
users:
- name: gitlab_com-1096606
  user:
    exec:
      apiVersion: client.authentication.k8s.io/v1
      args:
      - cluster
      - agent
      - get-token
      - --agent
      - "1096606"
      command: bin/glab
      env: null
      installHint: "To authenticate to the current cluster, glab is required. \n\nFollow
        the installation instructions at https://gitlab.com/gitlab-org/cli#installation.\n"
      interactiveMode: Never
      provideClusterInfo: false
- name: gitlab_com-1100000
  user:
    exec:
      apiVersion: client.authentication.k8s.io/v1
      args:
      - cluster
      - agent
      - get-token
      - --agent
      - "1100000"
      command: bin/glab
      env: null
      installHint: "To authenticate to the current cluster, glab is required. \n\nFollow
        the installation instructions at https://gitlab.com/gitlab-org/cli#installation.\n"
      interactiveMode: Never
      provideClusterInfo: false

Related Issues

Resolves #7448 (closed)

How has this been tested?

• Exploratory: see example kube config output above
• Unit tests as part of this change set

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap