Verifies the certificate authenticity.
The verify method retrieves the request timestamp and check if it falls in the limit set by the tolerance, by checking with the current timestamp. The verification follows the mechanism explained here : https://developer.amazon.com/docs/custom-skills/host-a-custom-skill-as-a-web-service.html#checking-the-signature-of-the-request
Request envelope of the input POST request in string format
Implementation of Verifier which provides a utility method to handle the request timestamp verification of the input request.