fix: StatelessRateDeserializer and WebhookDeserializer for underscore fields

[Justintime50]

Description

The removal of the serialized name fields was intentional in a previous PR; however, it was based on an assumption that the StatelessRate model should work like the rest of the models in this library. TLDR: The deserializer for that model did not properly map underscored JSON fields correctly which is why once removed the behavior exhibited like it did. The fix is to not return the serialized name fields but instead correct the StatelessRateDeserializer to work as expected. This bug report also surfaced that the WebhookDeserializer had the same issue and will be corrected as a part of this.

Testing

A/B tested with and without the fix, updated unit tests to protect against regressions

Pull Request Type

Please select the option(s) that are relevant to this PR.

• 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 not work as expected)
• Improvement (fixing a typo, updating readme, renaming a variable name, etc)