net: if: Connect interface with offloaded socket implementation

Instead of keeping a boolean informing whether a network interface is
offloaded at socket layer or not, keep a pointer to a function which
allows to create an offloaded socket. Native interfaces keep this as
NULL, while for offloaded interfaces it allows to connect an offloaded
socket implementation with an interface.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>