samples/zoap_server: Add support for the '/separate' resource

This resource is used to verify that the CoAP server is able to send
responses in two steps: 1. only acknowledge that the request was
received and is going to be handled; 2. The actual response, with the
payload.

Change-Id: Ia77cc0ee9805e6cc120c57f4598c68ad364882a0
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>