Add toString to graphqlCodegenStyle, Json

[dcsobral]

It would be helpful to sbt users if they could see what the value of these settings/tasks are, by turning the functions into classes defining toString.

[muuki88]

Thanks @dcsobral for the feature request. That makes totally sense. I guess we don't care about binary compatibility so we can just change the task type.

Would you like to open a pull request for this?

[dcsobral]

I don't think the task type needs changing at all. One is Function1, which can be extended by a class or object. The other is already a specific type, iirc.