// named with the signer name as a prefix (translating slashes to colons).
  // For example, for the signer name `example.com/foo`, valid
  // ClusterTrustBundle object names include `example.com:foo:abc` and
  // `example.com:foo:v1`.
  //
  // If signerName is empty, then the ClusterTrustBundle object's name must
  // not have such a prefix.
  //
  // List/watch requests for ClusterTrustBundles can filter on this field