op-mode: vrf: T5150: add "show vrf vni" and "show vrf <name> vni" commands

vyos@vyos:~$ show vrf vni
VRF VNI VxLAN IF L3-SVI State Rmac
blue 2000 None None Down None
green 3000 None None Down None
red 1000 None None Down None

vyos@vyos:~$ show vrf blue vni
VRF VNI VxLAN IF L3-SVI State Rmac
blue 2000 None None Down None

(cherry picked from commit 2fb763ffbc5c5babe552ec97c06570c54ea4aad8)