Add support for controlling area name

[cjakobsen]

Problem

The concept of an area in ATC generated code is used to control names of controllers, folders etc. This is today based solely on the root element of the api path. In some cases this is not ideal and this is a request to be able to control this.

An example is if multiple services are serving part of an api that share a common root element and it is actually a sub-element of the path that logically defines the area for each service.

Idea for solution

Be able to specify area name in the OpenApi file for each operation. Using the OpenApi spec for this will make it very clear how the area is linked to the generated code.

[MrAndersPedersen]

@cjakobsen to update issue and set up meeting to clarify further together with @davidkallesen and to be presented on a CodeJam session.