interfaces shouldn't need concrete implementations to be referred to in the schema

[schveiguy]

Example:

@GQLDUda(TypeKind.INTERFACE)
abstract class I
{
    string name;
}

class C1 : I
{
    long val;
}

class C2 : I
{
    string val;
}

struct SubType
{
    I getc1();
    I getc2();
}

struct Query
{
    SubType subType();
}

class Schema
{
    Query queryType;
}

If I do a query like:

query {
    subType {
      getc1 {
          ... on C1 {
              val
          }
    }
}

I get an error that type C1 is not recognized in the schema. I have to add some dummy values in the schema to get the introspection to work:

struct SubType
{
    NullableStore!C1 c1dontuse;
    NullableStore!C2 c2dontuse;
    I getc1();
    I getc2();
}

the library should provide a mechanism (probably a UDA) that adds concrete types for interfaces.