Name: device-group-definition
Type: request
Purpose: >
  Setup a new device group and add public key of first device
Response: device-group-details
Inherits: message
Elements:
  Signature: { required: true }
  From: { required: true }
  Device-Group-Id: { type: string, min_length: 32, max_length: 128 }
  Device-Name: { type: string, min_length: 3, max_length: 64 }
  Device-Public-Key: { type: public_key }
Rejection-Codes:
  22: Device group already exists
  31: Sender is not permitted to define device group