[kubectl-plugin] Use a more Golang-native approach to retrieve the CR status for testing

[machichima]

• Use a Golang-native way to get/create/delete some ray and core resources. (e.g. Ray().RayV1().RayClusters(namespace).List())

Why are these changes needed?

Currently, kubectl-plugin uses kubectl command to get results for testing. We should use a more Golang native solution to get K8s resources.

kuberay/kubectl-plugin/test/e2e/kubectl_ray_job_submit_test.go

Line 49 in 837c3fa

cmd = exec.Command("kubectl", "get", "--namespace", namespace, "rayjob", "rayjob-sample", "-o", "jsonpath={.status.jobId}")

Related issue number

Closes #3626

Checks

• I've made sure the tests are passing.
• Testing Strategy
  • Unit tests
  • Manual tests
  • This PR is not tested :(