Feature: generate datafetcher interfaces for java and kotlin via config

[mcarleio]

Add new configuration generateDataFetchersAsInterfaces and generation code for Java and Kotlin to generate data fetchers as interfaces (instead of generating data fetchers with some default example implementation).

❗ Based on #327

[mcarleio]

The following will show the usage on a small example:

Assume the following schema:

type Query {
  people: [Person]
}

type Person {
  name: String
  partner: Person
}

This will generate the following two interfaces:

public interface QueryDataFetcher {
  @DgsQuery(
      field = "people"
  )
  List<Person> people(DgsDataFetchingEnvironment dataFetchingEnvironment);
}

public interface PersonDataFetcher {
  @DgsData(
      field = "partner",
      parentType = "Person"
  )
  CompletableFuture<Person> partner(DgsDataFetchingEnvironment dataFetchingEnvironment);
}

The only open step is to implement these interfaces and add the @DgsComponent annotation:

@DgsComponent
public class QueryDataFetcherImpl implements QueryDataFetcher {
    @Override
    public List<Person> people(DgsDataFetchingEnvironment dataFetchingEnvironment) {
        // load persons e.g. from DB
    }
}

@DgsComponent
public class PersonDataFetcherImpl implements PersonDataFetcher {
    @Override
    public CompletableFuture<Person> partner(DgsDataFetchingEnvironment dataFetchingEnvironment) {
        // implement as needed (e.g. make a DB call, use a dataloader, ...)
    }
}