Add config options to add a prefix to generated types

[katmatt]

It would be very useful if our java code generator and our gradle plugin would allow to add a prefix to the names of all generated types.

Our use case is that we want to generate another and slightly different version of our model. And if we could add a prefix to the types of this model, it will be easier for us to distinguish these different types.

But I also think that this feature might be useful in other cases too. And I saw this pattern in other code generators too.