Calls SPIRE server `spire-server agent list` command

Example response

{
  "agents": [
    {
      "id": {
        "path": "/ns/default",
        "trust_domain": "example.org"
      },
      "attestation_type": "k8s_psat",
      "selectors": [
        {
          "type": "k8s_psat",
          "value": "agent_ns:spire"
        }
      ]
    }
  ]
}