Generate KUBECONFIG in KubernetesService#predefined_variables

What does this MR do?

Generate KUBECONFIG in KubernetesService#predefined_variables for runners.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests
  • Added for this feature/bug

What are the relevant issue numbers?

Closes #33360 (closed)