//  2. If it's a kubelet serving certificate, it is assigned
  //     "kubernetes.io/kubelet-serving".
  //  3. Otherwise, it is assigned "kubernetes.io/legacy-unknown".
  // Distribution of trust for signers happens out of band.
  // You can select on this field using `spec.signerName`.
  // +optional
  optional string signerName = 7;

  // expirationSeconds is the requested duration of validity of the issued

  // client needs to check the 'expiration' field in a response.
  // +optional
  optional int64 expirationSeconds = 4;

  // BoundObjectRef is a reference to an object that the token will be bound to.
  // The token will only be valid for as long as the bound object exists.
  // NOTE: The API server's TokenReview endpoint will validate the
  // BoundObjectRef, but other audiences may not. Keep ExpirationSeconds
  // small if you want prompt revocation.