feat(api): add JobService and ListClusters

[kkaneda]

This is mainly for debugging.

[kkaneda]

Example usage:

$ kubectl port-forward -n llmariner service/job-manager-server-grpc 8081 &

$ grpcurl -plaintext -H authorization:<token> -d "{}" localhost:8081  llmariner.jobs.server.v1.JobService/ListClusters
{
  "clusters": [
    {
      "id": "cluster-B_0YrG_sTdWabDitCB8I3zrn",
      "status": {
        "gpuNodes": [
          {
            "resourceName": "nvidia.com/gpu",
            "allocatableCount": 2
          }
        ]
      }
    }
  ]
}