Add ability to generate nullable fields wrapped to Optional

In my case I'm using client library to consume GraphQL API.
Server differentiate absents of the field in the query and explicitly passed null value.

I would like to have an ability to generate all nullable fields wrapped to an Optional.

So for the schema

type PersonUpdateInput {
    id: Int!
    name: String
}
type Mutation {
    updatePerson(patient: PersonUpdateInput!): Person!
}

The class will be generated like

class PersonUpdateInput {
  int id;
  Optional<String> name;
  
  setName(String name){
      this.name = Optional.ofNullable(name);
   }
}

This allow to explicitly set null value for 'name' field and server consume this value.

No file or test is named. Start by locating the generator entry point for nullable GraphQL input fields, then use the PersonUpdateInput example to trace how generated Java fields and setters are produced. Done means nullable fields can represent both absence and an explicitly supplied null through Optional.