feat(cluster): Implement basic `update-kubeconfig` command

Description

This change set implements a new command:

glab cluster agent update-kubeconfig --agent=<agent-id>

The is the one that a user interacts with to update a kubeconfig with a cluster, user and context to connect to the given agent.

There is documentation for this new command in Document GitLab CLI command usage to update kub... (gitlab!133438 - merged)

Related Issues

Part of #7296 (closed) #7297 (closed)

TODOs

• Get Implement REST API endpoint to create `k8s_prox... (gitlab!131923 - merged) merged
• Implement API from gitlab!131923 (merged) in go-gitlab
  • Draft PR: https://github.com/xanzy/go-gitlab/pull/1814
• Test e2e again

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