Computed columns on a Polymorphic Table
### Summary

I'm trying to attach a computed column to a polymorphic table with postgraphile 5.0.0-beta.20. The column only shows up on the specific types, but not the interface, so querying it requires a call-out for each type.

### Steps to reproduce

Create a minimal schema as follows:

```sql
BEGIN;

CREATE TYPE base_type AS ENUM ( 'A', 'B' );

CREATE TABLE base (
        id BIGSERIAL PRIMARY KEY,
        type base_type NOT NULL
);

COMMENT ON TABLE base IS $$
        @interface  mode:single type:type
        @type A name:VarA
        @type B name:VarB
$$;

CREATE FUNCTION base_common(b base)
RETURNS INTEGER
AS $$
        SELECT 42;
$$ LANGUAGE sql STABLE STRICT;

INSERT INTO base ( type ) VALUES ( 'A' );
INSERT INTO base ( type ) VALUES ( 'A' );
INSERT INTO base ( type ) VALUES ( 'B' );

COMMIT;
```

I launched postgraphile as follows:

```console
npx postgraphile --preset postgraphile/presets/amber --connection <redacted>
```

### Expected results

I expected this query to work, to allow me to access the `common` field.

```graphql
query {
  allBases {
    nodes {
      id
      type
      common
    }
  }
}
```

### Actual results

The above query caused the error message: `Cannot query field \"common\" on type \"Base\". Did you mean to use an inline fragment on \"VarA\" or \"VarB\"?`

I was able to access the `common` field with the following query. This is much more verbose, and makes adding additional polymorphic types a lot more difficult for my API consumers.

```graphql
query {
  allBases {
    nodes {
      id
      type
      ... on VarA {
        common
      }
      ... on VarB {
        common
      }
    }
  }
}
```

### Additional context

* Operating System: Linux (NixOS 23.11, but I doubt this is relevant).
* `node --version`: v18.18.2
* `postgres --version`: postgres (PostgreSQL) 15.5
* `postgraphile`: `5.0.0-beta.20`

Note: I've removed all the irrelevant parts of my schema to create a minimal working example. I understand that, as shown, there's no need for polymorphism, but it is required in my actual schema.

Thanks!