refactor and upgrade gNMI server
combine/refactor/upgrade the current gNMI server (copied from oc-freebsd-target) with the gNMI server written by Max Reinheimer
furthermore extend the gNMI server to be able to respond to all required gNMI functions (Get, Set, Subscribe, Capabilities)
(not necessarily fully and correctly, but at least a basic response and then we go from there)