Bluetooth: Mesh: Add model callback structure

Adds a structure of callbacks for each model instance. This allows for
more flexible model implementations, that can interact with the Mesh
stack without going through the application.

For now, only an init callback is added, replacing the init mechanism in
the foundation models. The init callback does not provide the primary
flag that used to be in the foundation model callbacks, but replaces
this with an inline function in access.h.

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