A unique UUID string per request, used for replay protection. Must not be reused within the timestamp window.
Regex: /\A[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}\z/i
Note: This header attribute is required when HTTP Request Signing is enabled for your account.