zebra: add ESI to the "show evpn mac vni <> mac <> json" output

zebra: add ESI to the "show evpn mac vni <> mac <> json" output

Sample output -
=============
anuradhak-VirtualBox# anuradhak-VirtualBox# show evpn mac vni 1000 mac 00:00:00:00:00:22 json

{
  "00:00:00:00:00:22":{
    "type":"remote",
    "remoteVtep":"0.0.0.0",
    "localSequence":0,
    "remoteSequence":0,
    "detectionCount":0,
    "isDuplicate":false,
    "syncNeighCount":0,
    "esi":"03:44:38:39:ff:ff:02:00:00:02", >>>>>>>>>>>>>>>>
    "neighbors":"none"
  }
}

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>