net: sockets_offload: Allow to enable/disable DNS offload at runtime

Add new socket offloading functions, allowing to enable/disable
offloaded DNS implementation at runtime. This may be useful if there is
a mix of offloaded/native network interfaces in the system, so the
application can choose which DNS backend to use.

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