bgpd: json cli output for bgp evpn overlay

bgpd: json cli output for bgp evpn overlay

This diff provides implementation for the cli:
"show bgp l2vpn evpn all overlay json"

Sample output after this change:
leaf-1# sh bgp l2vpn evpn all overlay json
{
  "bgpTableVersion":1,
  "bgpLocalRouterId":"10.100.0.1",
  "defaultLocPrf":100,
  "localAS":65000,
  "10.101.1.4:5":{
    "rd":"10.101.1.4:5",
    "[5]:[0]:[32]:[101.101.101.101]":{
      "prefix":"[5]:[0]:[32]:[101.101.101.101]",
      "prefixLen":288,
      "paths":[
        {
          "valid":true,
          "bestpath":true,
          "pathFrom":"external",
          "nexthop":{
            "ip":"10.100.0.2",
            "afi":"ipv4"
          },
          "overlay":{
            "esi":"00:00:00:00:00:00:00:00:00:00",
            "gw":"0.0.0.0",
            "rmac":"ea:47:79:75:22:1b"
          }
        },
        {
          "valid":true,
          "pathFrom":"external",
          "nexthop":{
            "ip":"10.100.0.2",
            "afi":"ipv4"
          },
          "overlay":{
            "esi":"00:00:00:00:00:00:00:00:00:00",
            "gw":"0.0.0.0",
            "rmac":"ea:47:79:75:22:1b"
          }
        }
      ]
    }
  },

...
...
}

Signed-off-by: Lakshman Krishnamoorthy <lkrishnamoor@vmware.com>