Describe the bug
Seems like turning on use_struct_references and setting pointer: optional breaks the ability to read from inline fragments.

To Reproduce
Problem happens at runtime.
Setting use_struct_references: true and pointer: optional leads to the removal of the interface(?) that is supposed to be usable for grabbing data from inline fragments.

In the docs design.md, there is a block of code similar to

if pm, ok := transaction.PaymentMethod.GetDetails().(destination.ChargeCreditCardChargeCreditCardTransactionPayloadTransactionPaymentMethodDetails); ok {
}

While this code doesn't throw a compiler error with the default config, when you adjust the config vars mentioned above, doesn't seem like there is a way to get the inline fragment data. Messages similar to invalid operation: transaction.PaymentMethod.GetDetails() (value of type *destination.ChargeCreditCardChargeCreditCardTransactionPayloadTransactionPaymentMethodDetails) is not an interface pop up.

I am using this library to query/send mutations to Braintree. The reason I set the config vars above is primarily because Braintree's API doesn't handle Golang default values for Braintree's GraphQL inputs.

Here's what this mutation looks like:

mutation ChargeCreditCard(
  $input: ChargeCreditCardInput!) {
  chargeCreditCard(input: $input) {
    transaction {
      id
      legacyId
      status
      createdAt
      paymentMethod {
        id
        details {
          __typename
          ... on CreditCardDetails {
            cardholderName
            last4
            brandCode
            expirationMonth
            expirationYear
          }
        }
      }
    }
  }
}

Expected behavior
Generating code in such a way that this behavior works. Unless I'm doing something incorrectly/sub-optimally?

genqlient version
main

Additional context
Add any other context about the problem here.