fix: upgrade server algorithm validation

[doiron]

Description

Upgrading the ASK SDK Java Servlets skill request signature verification handler to use the SHA256withRSA signature algorithm using the Signature-256 request header in order to validate the request is a valid Alexa HTTP requests.

Motivation and Context

This change is part of a multi-phase effort to move off of SHA-1 algorithm for all tls usage.

Testing

Screenshots (if appropriate)

Types of changes

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

Checklist

• 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
• I have added tests to cover my changes
  Current tests are leveraging the constants that have been updated in this PR so no changes made
• All new and existing tests passed

License

• By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.