[client] support query directives

[dariuszkuc]

Discussed in #1281

^{Originally posted by FidasL October 25, 2021}
Hi!
I have custom graphql-server and graphql-client both developed with graphql-kotlin library. On the client side, I want to use schema directives on topLevelObject:

query {
  students @include(if: false) {
    id
    firstName
    lastName
  }
}

But after execute query like this from client, request serialization fails, because it generates non-nullable types in Result class:

  public data class Result(
    public val students: List<Student> // -> this type should be nullable
  )

Please, is this an issue, or do I need to configure something differently? Our client version is 4.1.1. Thank you :)