A type does not implement an interface with a complex field

@MichalLytek is already working on this.

Since Jun 3, 2021.

**Describe the Bug**
The generated schema does not properly handle the inheritance of interfaces having a non-scalar field. More concretely, an object type declared to implement an interface does not show up as an implementation in the schema.

**To Reproduce**
A repo with a reproducible example: https://github.com/dzhelezov/type-graphql-interface-example
Run with `yarn ts-node src/index.ts` and inspect `src/schema.gql`:
```gql
type EventImpl {
....
}
```

**Expected Behavior**
(Referring to the repo above): The generated schema includes `type EventImpl implements IEvent`

**Environment (please complete the following information):**

- OS: [MacOS]
- Node: v12.19.0
- Package version: 1.1.1
- TypeScript version: 3.9.9