zebra:add df flag into evpn esi json output

zebra:add df flag into evpn esi json output

FRR "show evpn es 'esi-id' json" output dont have the 'df' flag.

Modified the code to add the 'df' flag into json output.

Before Fix:

```
torm-11# show evpn es 03:44:38:39:ff:ff:01:00:00:01 json
{
  "esi":"03:44:38:39:ff:ff:01:00:00:01",
  "accessPort":"hostbond1",
  "flags":[
    "local",
    "remote",
    "readyForBgp",
    "bridgePort",
    "operUp",
    "nexthopGroupActive"
	 ====================> df is missing
  ],
  "vniCount":10,
  "macCount":13,
  "dfPreference":50000,
  "nexthopGroup":536870913,
  "vteps":[
    {
      "vtep":"27.0.0.16",
      "dfAlgorithm":"preference",
      "dfPreference":32767,
      "nexthopId":268435460
    },
    {
      "vtep":"27.0.0.17",
      "dfAlgorithm":"preference",
      "dfPreference":32767,
      "nexthopId":268435461
    }
  ]
}
torm-11#
```

After Fix:-

```
torm-11# show evpn es 03:44:38:39:ff:ff:01:00:00:01 json
{
  "esi":"03:44:38:39:ff:ff:01:00:00:01",
  "accessPort":"hostbond1",
  "flags":[
    "local",
    "remote",
    "readyForBgp",
    "bridgePort",
    "operUp",
    "nexthopGroupActive",
    "df" ========================> designated-forward flag added
  ],
  "vniCount":10,
  "macCount":13,
  "dfPreference":50000,
  "nexthopGroup":536870913,
  "vteps":[
    {
      "vtep":"27.0.0.16",
      "dfAlgorithm":"preference",
      "dfPreference":32767,
      "nexthopId":268435460
    },
    {
      "vtep":"27.0.0.17",
      "dfAlgorithm":"preference",
      "dfPreference":32767,
      "nexthopId":268435461
    }
  ]
}
torm-11#

```

Ticket:# 3447935

Issue: 3447935

Testing: UT done

Signed-off-by: Sindhu Parvathi Gopinathan's <sgopinathan@nvidia.com>
Signed-off-by: Chirag Shah <chirag@nvidia.com>