net: lwm2m: Add config to enable DNS support

Add new config option `LWM2M_DNS_SUPPORT` to the LWM2M library, instead
of relying on `DNS_RESOLVER` which is only compatible with native
network stack. This allows to use DNS with offloaded interfaces
seamlessly.

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