[enhancement] don't make abstract classes and embeddables api resources

Right now if I have this `schema.yml`

```yaml
types:
Thing:
properties:
name: ~

Person:
properties:
familyName: ~

Country:
parent: false
embeddable: true
properties:
address: { range: "Text" }

PostalAddress:
parent: false
properties:
addressCountry: { range: "Country", embedded: true }
```

It will generate all the entities and all entities are marked as an api resource in the class annotations. The only problem is that you don't want `Thing` and `Country` to be api resources. `Thing` is an abstract class so it can't even be initialized. And since `Country` is an embeddable it isn't linked to any database table so you can never fetch any countries from the api.