Make signature verifier more resilient to HTTP retrieval errors

[breedloj]

Description

Certificate retrieval logic will now retry up to 5 times in cases where:

• A non-200 response code is received
• An IOException occurs when establishing the HTTP connection

Additionally, the cert will be checked for validity (not expired) before re-using it from the cache, avoiding a possible edge case issue if a long-running instance overlapped two certificates.

Testing

Tested with a servlet enabled sample skill in various conditions.

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
• All new and existing tests passed

License

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

[nikhilym]

LGTM 🚢