[feature] Allow showing variables devices inherit from related objects in REST API

[nemesifier]

The admin preloads the template default values, group variables, org variables and global variables so the user can edit them easily.

The REST API does not provide the same mechanism.

I think we can add either one or multiple properties showing these variables (separated from the context of the device).
I think we should make this optional, it could be enabled with a GET parameter.