feat(rate): add details field for dynamic metadata

[lanej]

Summary

Adds an optional details field to the Rate model to support dynamic metadata in API responses. This field enables flexible, feature-flag-driven data for specific carriers and shipment options, starting with Amazon's USPS Ship API integration.

Fixes SHPE-727

Changes

• Add details field of type Map<String, String> to Rate class
• Field is optional and null when not present in API responses
• Fully backward compatible with existing code

API Example

{
  "id": "rate_...",
  "carrier": "USPS",
  "service": "Priority",
  "rate": "7.33",
  "details": {
    "some_string_key": "value",
    "another_key": "value"
  }
}

Access in Java:

Rate rate = shipment.getLowestRate();
Map<String, String> details = rate.getDetails(); // null if not present

Testing

• All existing RateTest tests pass
• Gson automatically handles serialization/deserialization
• Lombok @Getter generates accessor method

Notes

Release constraint: Cannot be released before 2025-12-09 per Amazon integration timeline.