Provide support for type augmentation

[dunqan]

We should allow for the extensibility of our build-in types. Adding more properties for core models helps in implementing a data-driven approach for both our own libraries and 3rd party ones.

There are some well-known limitations in TypeScript around it, mentioned in those tickets:

• Interface merge of import/local should error when the import is re-exported with export * microsoft/TypeScript#9532
• Augmenting external module that is re-exported? microsoft/TypeScript#18877

The "plan of the attack" is:

1. Expose model we want to allow customers to augment in our public-api.ts
2. Create an additional build step for our libraries that will move those model also to main entry point generated by ng-packagr (eg spartacus-core.d.ts for core). It could be done by some additional utility script that will run after the build, by creating new builder extending default one or bay using some additional tools (like: https://www.npmjs.com/package/ng-cli-packagr-tasks)

[dunqan]

QA steps:

1. Import and Augment Product model (in app.module for example):

import { Product } from '@spartacus/core';

/...

declare module '@spartacus/core' {
  interface Product {
    custom?: string;
  }
}

2, Confirm that augmenttion works (there is no compilation errors) for both dev and prod builds:

const p: Product = {
  code: '234',
  custom: '43432',
};

console.log(p.code);
console.log(p.custom);

[giancorderoortiz]

Would like to test this one :-)

[giancorderoortiz]

Test successful