Backport GraphQLUnion to 4.x.x

[smyrick]

📝 Description

Since the schema can be generated from code you can use existing Kotlin classes from common libraries to define shared models. However if you want to declare an object defined in the library as implementing some new interface or union in your service it is not possible, since we can't modify the class definition to add Kotlin code to it.

These changes would allow us to use Any as the return type in the schema but then, through annotations, modify the schema return type to match the new class we want to use.

For example in an SDL-First world you could have this Kotlin class defined in some library

data class SharedModel(val foo: String)

And then write your schema as the following

interface CustomServiceInterface {
  foo: String!
}

type SharedModel implements CustomServiceInterface {
  foo: String!
}

type CustomServiceModel implements CustomServiceInterface {
  foo: String!
  bar: String!
}

union CustomServiceUnion = SharedModel | CustomServiceModel

But this is not currently possible in the full code-generation approach

🔗 Related Issues

PR into master branch: #1194

[dariuszkuc]

Can you update 4.x docs as well?