feat: add retrieve_stateless_rates function

[Justintime50]

Description

Adds new retrieve_stateless_rates function that takes in params like you would with shipment creation under the beta namespace.

Testing

Added a new test to ensure this works, inspected the cassette

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)