Add support for VPP API that are alternative to:
- set/show interface nat64
- nat64 add pool / show nat64 pool
- nat64 add static bib/show nat64 bib
nat-instance - vrf mapping can be preserved
VPP uses different apis for nat44 and nat64.
Ietf-nat model unifies the api, but it is enough to augment external-ip-address-pool needs to
capture all required information.