Bluetooth: Mesh: Add persistent model user data

Adds user data file in the model settings, allowing each model instance
to store some application specific data without having to reinvent the
settings path encoding for model element/ID combinations. Exposes the
settings_handler interface in the model callback structure and adds a
data store function.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>