Schema directives support

Nobody has claimed this yet.

## Partially solved by https://github.com/MichalLytek/type-graphql/pull/369

### Currently blocked by https://github.com/graphql/graphql-js/issues/1343

---

Ability to define any arbitrary graphql directive to be passed to the compiled schema:

```ts
@ObjectType({ directives: ['@cacheControl(maxAge: 240)'] })
class SampleObject {
// ...
}
```

Which should be equivalent to:
```graphql
type SampleObject @cacheControl(maxAge: 240) {
# ...
}
```

It might be also implemented as a separate decorator that would make possible to create custom decorators like `@CacheControl({ maxAge: 240 })`:
```ts
@Directive('@cacheControl(maxAge: 240)')
@ObjectType()
class SampleObject {
// ...
}
```

Begin with PR #369 and graphql/graphql-js issue #1343; they identify the existing partial work and external blocker. Once unblocked, completion means supporting arbitrary directives on the compiled schema, including the illustrated ObjectType option or Directive decorator behavior.

Written by the indexing model from the issue text.