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• NOT_CONTROLLER as we are only forwarding admin write requests.
• CLUSTER_AUTHORIZATION_FAILED if the inter-broker verification failed.

The CLUSTER authorization for EnvelopeRequest takes place during the request handling, similar to LeaderAndIsrRequest. This ensures the EnvelopeRequest is not sent from a malicious client pretending to be a fellow broker. Once that part is done, we shall replace the request context with Principal information embedded inside the EnvelopeRequest to complete the inner request permission check. For inner request error, it will still be embedded inside the `ResponseData` struct defined in EnvelopeResponse below.

{
  // Possible top level error code:
  //
  // NOT_CONTROLLER
  // CLUSTER_AUTHORIZATION_FAILED
  //
  "apiKey": N,
  "type": "response",
  "name": "EnvelopeResponse",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "ResponseHeader", "type": "Header", "versions": "0+",
      "about": "The embedded response header." },
	{ "name": "ResponseData", "type": "ApiMessage", "versions": "0+",
      "about": "The embedded response data."},
    { "name": "ErrorCode", "type": "int16", "versions": "0+",
      "about": "The error code, or 0 if there was no error." },
  ]
}

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