Add support for custom model addapters

[kito99]

I'm customizing the template in order to change how the Java code is generated. However, there are a couple of things I'd like to do that can't be done without modifying STDModel.java (which I would like to avoid doing). I noticed that one way around this is to use StringTemplate model adaptor (https://github.com/antlr/stringtemplate4/blob/master/doc/adaptors.md). It'd be great if this project had support for adding custom model generators; it would make it quite extensible.

[brimworks]

I'm not following on how model adaptors would be useful. Do you have some example code?

[kito99]

I don't have example code, but the scenario is pretty simple. I'm just trying to determine if a particular type is a Query. I don't see that information in the GraphQL model for the AST, but I could do something silly like check the name of the type or look for a special comment. However, since StringTemplate doesn't allow arbitrary expressions, the only option is to call a method on STDModel that returns a boolean value. Rather than modifying the class directly, if I could register a model adapter, I could add a new boolean value like isQuery that I could use in the template.

[brimworks]

Ah, I think I understand... do you want to add an isQuery() method to the STModel? We always appreciate pull requests :).

[kito99]

Well, adding the model adapter would allow extending the model in ways that aren't generally useful to other teams. In this case, though, is there any way to tell if a type is a query other than looking at the name or a comment?