Support for Optional when returning nullable types

[paul-pop]

Thank you for the library, it's great! 👏

Is your feature request related to a problem? Please describe.
Because the query resolvers return the nullable object (without wrapping it in Optional) it can feel outdated for people heavily using Java8+ features.

Describe the solution you'd like
Currently when you have a query that has a nullable return type like this:

type Query {
    user(id: ID!): User
}

the generated code is:

@Override
public User user(final String id) {
    return null;
}

Would be nice to have a flag to enable/disable behaviour that allows the generation for nullable return types to use Optional instead which can make it easier to write neater code in the resolver.

@Override
public Optional<User> user(final String id) {
    return Optional.empty();
}

Describe alternatives you've considered
None

Additional context
None

[kobylynskyi]

@paul-pop thanks for reporting this feature request.
I've added a new boolean config useOptionalForNullableReturnTypes to maven and gradle plugins: #274
Will be released this week as part of version 3.0.0.

@jxnu-liguobin I've created a separate ticket for you to work on sbt plugin changes: #276

[kobylynskyi]

@paul-pop Version 3.0.0 was released today.