net: if: Add function for network interface offloading status

Add new net_if_is_offloaded(iface) that returns true if
the network interface is offloaded (either IP or socket
offloading is enabled for that interface) and false if
the interface is not offloaded.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>