Bluetooth: Mesh: Expose destination address in bt_mesh_msg_ctx

There are valid use cases where the model layer must know the true
destination address. So far only the fact that it was one of the
addresses that the model subscribes to (its element's unicast
included) has been knowable.

Solve the issue by moving the destination address from the internal
net_rx context to the public bt_mesh_msg_ctx struct.

Fixes #7453

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>