Add ability to make Java models extend a class and implement interfaces.

[RicoYao]

Example:
plank --java_extends=BaseModel --java_implements="ModelInterfaceA,ModelInterfaceB"

Generates:
class Board extends BaseModel implements ModelInterfaceA, ModelInterfaceB { ...

In another diff, I'm adding the ability to add custom annotations and imports, so developers can also add @Override and import the extended class and implemented interfaces if necessary.

[rahul-malik]

Can we discuss this particular solution a bit more and its purpose? Sorry if we've covered this in another thread (it's been a while!). It seems a bit strange to have this be a global configuration setting IMO and I'd like to make sure we're going down a path that would be suitable as plank adopts more languages

[RicoYao]

Sure, perhaps we can work off of a specific example:

The Plank-generated model generates a mergeFrom method. We may need to call that method without knowing the specific model type in layers of our codebase that deal with Models in a more generic fashion (e.g. persistence layers). So my thought is to allow for defining an interface that the Plank-generated model happens to implement. We may also need something similar to get the ID of the model.

Model.java (not Plank-generated)

interface Model {
  public String getId();
}

MergeableModel.java (not Plank-generated)

interface MergeableModel<M: Model> {
  public M mergeFrom(M model);
}

ModelStorage.java (Not Plank-generated. This isn't really how our code is set up, it's just an example.)

class ModelStorage {
  public void updateModelStorage(Model model) {
    // Merge fields from previous version of the model before updating the storage layer
    if (model instanceof MergeableModel) {
      MergeableModel oldModel = getModelFromStorage(model.getClass(), model.getId());
      model = oldModel.mergeFrom(model);
    }
    storeModel(model);
  }
}

Board.java (Plank-generated)

public class Board implements MergeableModel<Board> {
  @Override
  public Board mergeFrom(board) { ... }

  @Override
  public String getId() { ... }
}

In terms of this being a "global configuration", are you referring to it applying to other languages? I had implemented it as a Java-specific config parameter (--java_extends and --java_implements) as I wasn't confident it would translate easily to other languages.

[RicoYao]

Replaced by #217