Model extensions interface

[ridafkih]

This PR adds the ability to use the extendModel function to extend the model to include the attributes of the extended model provided into the function.

Extending Models

Sometimes one model is derivative of another, and the extension feature can be quite useful for this.

If we want to extend the PostModel to create an ImagePostModel which has all its old properties but with an imageUrl string column, we can do so without having to re-write every single other attribute.

// ./models/ImagePostModel.model.ts

import { extendModel } from "schemix";

import PostModel from "./Post.model";

export default extendModel(PostModel, (ImagePostModel) => {
  ImagePostModel.string("imageUrl");
});