[NSHSFC-27] Add NSH into per-packet metadata

[vvalderrv]

To enable VPP based network function (e.g. SNAT, load balancer) to be real NSH-aware, it needs the NSH_SFC plugin to save the parsed NSH to the per-packet metadata for those network function to consume.

The initial support by collocating NSH Proxy with network function is not the real solution, because the NSH is invisible to network function.