Support ADR pattern in subscriptions

@MichalLytek is already working on this.

Since Jun 10, 2021.

**Is your feature request related to a problem? Please describe.**
When using the Subscription annotation with a custom subscribe, you cant access your class dependencies and cant use the parameter annotations to selectively pick from the resolver args.

**Describe the solution you'd like**
A solution would be to allow placing the Subscription annotation on a class (and maybe also the FieldResolver annotation later) enabling the ADR pattern. Translated to graphql this would make, the Subscription class the Action, the Domain are your (internal) services & models and the Responder is graphql itself.

```ts
@Subscription(() => News) // or its own dedicated decorator SubscriptionAction
export class NewsSubscription implements SubscriptionAction {
public constructor(/* my deps */) {}

// required if no string topics given, overrides declaritive options from annotation if given
public subscribe(@Arg('category') category: string): AsyncIterator | Promise {
// ...
}

// optional, args get merged
public resolve(@Arg('language') language: string): Promise {
// ...
}
}
```

**Additional context**
https://en.wikipedia.org/wiki/Action%E2%80%93domain%E2%80%93responder
It can also improve reuseability and organization.