Provide cardinality annotations

[ramnivas]

Currently, both side of a relationship need to refer to the other side. However, this can be an issue when using (for example) an independently developed exo model. We can offer an annotation such as @manyToOne, @oneToMany, @oneToOne.

An additional semantics we can deduce is that unless a field is explicitly specified, we won't offer it in the return value. For example, in the code below, there will be no GraphQL field named concerts in the Venue type.

@postgres
service ConcertService {
  type Concert {
    ...
    @column("venueid") @manyToOne venue: Venue
  }

  type Venue {
    ...
    // NOT NEEDED @column("venueid") concerts: Set<Concert>?
  }
}