Bluetooth: Mesh: add extension config server by private beacon server

Commit add extension of configuration server by private
beacon server.
Mesh Protocol v1.1 specification
4.4.11.1
The Mesh Private Beacon Server model is a main model
that extends the Configuration Server model.

Configuration server and private beacon server shall always
exist together since they have bound states.

Signed-off-by: Aleksandr Khromykh <aleksandr.khromykh@nordicsemi.no>