Immutability

I’d like the DTOs generated by graphql-java-codegen to optionally be “immutable”.

By “immutable” I mean that the generated classes do not have any setters. Very similar to to what Lombok does with its @Value annotation.

A generated class with this feature enabled should look like:

public class Person {
    // Everything as usual, except...
    // NO SETTERS IN HERE
}

If you approve the addition of this feature, I am happy to implement and PR it.

[lorenzleutgeb]

Just to confirm: Starting from version 2, together with the feature described in #148, MapStruct can now target immutable DTOs and will instantiate them via the builder 🎊

[lorenzleutgeb]

I think that generateImmutableModels (verb, adjective, noun) better fits other, already existing, names such as generateAsyncApi, so I’ll choose that.