Add special timestamp verification for skill event requests

[breedloj]

Description

Decouples the verification tolerances for standard skill requests and skill event requests which have different maximums.

Motivation and Context

Due to the delivery characteristics of skill event requests, these requests may arrive with a timestamp value of up to 1 hour prior to the current time and are verified against a different upper bound value than standard requests.

Standard requests should be verified within 150 seconds. This change splits the verification logic of the two request classes.

https://developer.amazon.com/en-US/docs/alexa/smapi/skill-events-in-alexa-skills.html#delivery-of-events-to-the-skill

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [x] My code follows the code style of this project
• [ ] My change requires a change to the documentation
• [ ] I have updated the documentation accordingly
• [ ] I have read the README document
• [x] I have added tests to cover my changes
• [x] All new and existing tests passed

License

• [x] I confirm that this pull request can be released under the Apache 2 license