Provide a way to select an `<api>` from a pack without implementation

[MiloradCvjetkovic]

Today the selection of an <api> [-version] is implicit through the selection of a component implementing an <api> and the component's attribute Capiversion.

For building libraries or project local implementation of an <api> it is useful to be able to reference just the API from a pack and not to require a component e.g. Custom or similar that does not contain any implementation (or just a template for the implementation).

[ReinhardKeil]

This may require an extension to the csolution tool.

The current solution is to provide a "custom" component that provides access to API headers and user code templates for implementation. IMHO this could be also used for the library variant of a middleware component that needs to compile against the API.

Let's discuss how this would be used in a practical implementation.

[ReinhardKeil]

This affects developers that provide a standardized middleware. Compared to users of this middleware it is only a very small fraction of developers. Making API a selectable option (i.e. in an IDE) it could get confusing to middleware users.

Work-arounds for middleware developers are:

• use "custom" component (with empty implementation) to access the API.
• use explicit paths to the API for translating the library, should be not a problem for libraries that expose their own APIs.

[ReinhardKeil]

Add hints on how to create middleware packs in a documentation