Update a forwarding rule

{
  "success": true,
  "result": {
    "uuid": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
    "localpart": "<string>",
    "destination": "*@theirinbox.com",
    "last_modified": 123,
    "created_at": 123
  }
}

{
  "success": false,
  "error": "localpart cannot be changed; delete the rule and create a new one to rename it"
}

{
  "success": false,
  "error": "User information not found"
}

{
  "success": false,
  "error": "Forwarding rule not found"
}

{
  "success": false,
  "error": "Another forwarding rule already exists with this localpart and destination"
}

{
  "success": false,
  "error": "Failed to update forwarding rule"
}