samples/zoap_server: Adds example for the link-format feature

This implements the /.well-known/core resouce and two children
resources (/core1 and /core2) so the link-format feature is better
explained.

Change-Id: I9dd8c69040c952c5d12a9987c1966a71b0257ef2
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>