GraphQL interfaces not unmarshaled by the test client

### What happened?

I got an error when decoding a response for a type with an interface property using the test client.

### What did you expect?

I expected it to work, handling the parsing of an interface to the corresponding type.

### Minimal graphql.schema and models to reproduce

Schema and model.

```graphql
type Example {
foo: FooInterface
}

type FooInterface {
version: String!
}

type FooInterfaceV1 {
version: String!
someField: String!
}

extend type Query {
getExample(): Example!
}
```

Query.

```graphql
query {
getExample() {
foo: {
__typename
... on FooInterfaceV1 {
someField
}
}
}
}
```

Error `'getExample.someField' expected type 'FooInterface', got 'map[string]interface {}'`

This is caused by the interface being generated as a `interface{}` which with JSON (and I assume map structure) will be parsed as `map[string]interface{}`. There's no custom unmarshaling generated to properly support interfaces it seems.

### versions
- `go run github.com/99designs/gqlgen version` - v0.17.20
- `go version` - go version go1.19.3 darwin/amd64