Extending a type with extra fields (without adding them to the class directly)

[terminalnode]

Lets say I have a type:

type MyType @key(fields: "id ownerId") @sharable {
  id,
  ownerId,
  fieldA,
  fieldB,
  fieldC
}

This type is shared between ServiceA and ServiceB. I can add service specific fields to it by creating this class in ServiceA:

@ShareableDirective
@GraphQlName("MyType")
@KeyDirective(fields = FieldSet("id ownerId"))
class MyTypeGql(
  val id: Long,
  val ownerId: Long,
  val fieldA: String,
  val fieldB: String,
  val fieldC: String,
) {
  fun serviceSpecificField() = TODO()
}

and adding a federated type resolver, so that when ServiceB emits a MyType the router will fetch serviceSpecificField from ServiceA.

I have something like this for a relatively simple class and it seems to work well, but now I want to add it to a class that has something like ~30 fields with a lot of directives and such on top. Duplicating this class to both services is bit of a pain, and keeping them in sync is even worse.

If I could just define an extension method:

fun MyType.serviceSpecificField() = TODO()

Or if I could define some type of standalone field resolver:

class MyTypeServiceSpecificFieldResolver() : FieldResolver<MyType> {
  override val name = "serviceSpecificField"
  
  override val resolve(
    environment: DataFetchingEnvironment,
    entity: MyType,
  ) = TODO()
}

or something like that. This would also be handy for non-federated types with a lot of fields, to keep them clean and perhaps be able to unit test them individually.