fix(federation): Handle ObjectTypeExtension

[timhall]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

From the federation example, the following invalid typings are generated:

type Post @key(fields: "id") {
  id: ID!
  title: String!
  body: String!
  user: User
}

extend type User @key(fields: "id") {
  id: ID! @external
  posts: [Post]
}

extend type Query {
  getPosts: [Post]
}

export class Post {
  id: string;
  title: string;
  body: string;
  user?: User
}

Where a User class/interface is not generated

Issue Number: N/A

What is the new behavior?

This PR handles ObjectTypeExtension the same was as ObjectTypeDefinition when generating Typescript definitions.

export class Post {
  id: string;
  title: string;
  body: string;
  user?: User
}

export abstract class IQuery {
    abstract getPosts(): Post[] | Promise<Post[]>;
}

export class User {
    id: string;
    posts?: Post[];
}

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[kamilmysliwiec]

Thank you!