[catnip] Add Catnip (DPDK) basic usage examples

[kiruchon]

Issue

There are no clear examples of how to use catnip (DPDK) on demikernel/examples/*
Firstly, I thought they are compatible with existing examples that work on catnap.
But when switching to catnip and trying to use any of these examples, it throws an error "Unsupported LibOS"

Solution

Add Catnip (DPDK) basic TCP and UDP usage examples to examples/*