Facilitate backend integration in addons

[sinedied]

In order to facilitate the creation of backend-oriented addons to make the generator fullstack, we should prepare some groundwork.

Tasks

• Add core constructor option to classify the kind of generator being made: client, server or fullstack. This will determine where the generator files shoud go and enable "fullstack" mode when one addon is either client or server. For fullstack generator, 2 separate template folder should be provided for client and server. Default to client mode.
• Allow to configure destination path for frontend and backend folders (defaults: client / server) using a shared prop.