// using a `spec.signerName=NAME` field selector.
  //
  // +optional
  optional string signerName = 1;

  // trustBundle contains the individual X.509 trust anchors for this
  // bundle, as PEM bundle of PEM-wrapped, DER-formatted X.509 certificates.
  //
  // The data must consist only of PEM certificate blocks that parse as valid
  // X.509 certificates.  Each certificate must include a basic constraints