List fees

curl --request GET \
  --url https://sandbox-api.billingrails.com/v1/fees \
  --header 'Authorization: Bearer <token>'

{
  "fees": [
    {
      "object": "fee",
      "id": "fee_IhMUl3rrZ3",
      "created_at": "2025-02-14T17:33:40.843Z",
      "meter_id": "meter_IhMUl3rrZ3",
      "plan_id": "plan_IhMUl3rrZ3",
      "name": "Basic Plan",
      "code": "basic_plan",
      "description": "Basic plan for users",
      "invoice_name": "Basic Plan",
      "model": "standard",
      "currency": "NGN",
      "interval": {
        "unit": "month",
        "frequency": 1
      },
      "billing_cycles": 1,
      "amount": 1000,
      "package_size": 2,
      "unit_label": "user",
      "free_units": 0
    }
  ]
}