Bug fixes related to using --generateInterfaces config option for data classes.

[srinivasankavitha]

1. Ensure data classes that have interface fields return the appropriate interface type in getters.
2. Also ensure generated interface classes for corresponding data classes contain the original super interfaces .

For this schema,

interface Fruit {
    name: String
}
        
type Apple implements Fruit {
    name: String
}
        
type Basket {
  fruit: Fruit
}

the generated IApple interface did not extend Fruit . Also in Basket, the return type of getFruit was IFruit instead of Fruit where IFruit is not generated because it is already an interface.

This PR fixes both these issues.