// Introduced for KIP-590, EnvelopeRequest is what brokers use to wrap an
// incoming request before forwarding it to another broker.
EnvelopeRequest => key 58, max version 0, flexible v0+, admin
  // The embedded request header and data.
  RequestData: bytes
  // Value of the initial client principal when the request is redirected by a broker.
  RequestPrincipal: nullable-bytes
  // The original client's address in bytes.
  ClientHostAddress: bytes

EnvelopeResponse =>
  // The embedded response header and data.
  ResponseData: nullable-bytes
  // The error code, or 0 if there was no error.
  //
  // NOT_CONTROLLER is returned when the request is not sent to the controller.
  //
  // CLUSTER_AUTHORIZATION_FAILED is returned if inter-broker authorization failed.
  ErrorCode: int16