Name: attendee-count-request
Type: request
Purpose: >
  Get number of badges in a specific event session
Response: attendee-count-results
Inherits: message
Elements:
  Signature: { required: true }
  From: { required: true }
  Event-Id: { required: true, type: identifier, class: event }
  Session-Id: { type: string, min_length: 3, max_length: 32 }
  Deadline: { type: timestamp }
Rejection-Codes:
  22: Event does not exist
  31: Sender is not authorized to count attendees