Add undocumented rate_details field to typings

[jakeleventhal]

The rate response object contains an undocumented rate_details field. This PR adds typings for it. Here is an example of how it looks in the API:

"rate_details": [
  {
    "rate_detail_type": "location_fee",
    "carrier_description": "Residential Address",
    "carrier_billing_code": "270",
    "carrier_memo": null,
    "amount": {
      "currency": "usd",
      "amount": 6.55
    },
    "billing_source": "Carrier"
  },
  {
    "rate_detail_type": "shipping",
    "carrier_description": "Shipping",
    "carrier_billing_code": "BaseServiceCharge",
    "carrier_memo": null,
    "amount": {
      "currency": "usd",
      "amount": 128.69
    },
    "billing_source": "Carrier"
  },
  {
    "rate_detail_type": "fuel_charge",
    "carrier_description": "Fuel Surcharge",
    "carrier_billing_code": "375",
    "carrier_memo": null,
    "amount": {
      "currency": "usd",
      "amount": 22.99
    },
    "billing_source": "Carrier"
  }
]