vrrpd: add priority field into interface json

vrrpd: add priority field into interface json

'Priority' attribute is missing in "show vrrp interface <intf> json"
output. Whereas it is there in non-json output.

It has been added now in show vrrp interface json output.

Before Fix:

```
vrrp1# show vrrp interface swp5.101 json
[
  {
    "vrid":2,
    "version":3,
    "autoconfigured":false,
    "shutdown":false,
    "preemptMode":true,
    "acceptMode":true,
    "interface":"swp5.101",
    "advertisementInterval":1000,
    "v4":{
      "interface":"vrrp4-11-2",
      "vmac":"00:00:5e:00:01:02",
      "primaryAddress":"50.0.0.2",
      "status":"Master",
      "effectivePriority":110,
      "masterAdverInterval":1000,
      "skewTime":570,
      "masterDownInterval":3570,
      "stats":{
        "adverTx":248456,
        "adverRx":1,
        "garpTx":1,
        "transitions":2
      },
      "addresses":[
        "50.0.0.1"
      ]
    },
    "v6":{
      "interface":"vrrp6-11-2",
      "vmac":"00:00:5e:00:02:02",
      "primaryAddress":"fe80::7f1:49e7:768c:aa73",
      "status":"Master",
      "effectivePriority":110,
      "masterAdverInterval":1000,
      "skewTime":570,
      "masterDownInterval":3570,
      "stats":{
        "adverTx":248455,
        "adverRx":1,
        "neighborAdverTx":1,
        "transitions":2
      },
      "addresses":[
        "2001:50::1"
      ]
    }
  }
]
vrrp1#
```

After Fix:

```
vrrp1# show vrrp interface swp5.101 json
[
  {
    "vrid":2,
    "version":3,
    "autoconfigured":false,
    "shutdown":false,
    "preemptMode":true,
    "acceptMode":true,
    "interface":"swp5.101",
    "advertisementInterval":1000,
    "priority":110, ====> priority added into json output
    "v4":{
      "interface":"vrrp4-11-2",
      "vmac":"00:00:5e:00:01:02",
      "primaryAddress":"50.0.0.2",
      "status":"Master",
      "effectivePriority":110,
      "masterAdverInterval":1000,
      "skewTime":570,
      "masterDownInterval":3570,
      "stats":{
        "adverTx":15,
        "adverRx":4,
        "garpTx":1,
        "transitions":2
      },
      "addresses":[
        "50.0.0.1"
      ]
    },
    "v6":{
      "interface":"vrrp6-11-2",
      "vmac":"00:00:5e:00:02:02",
      "primaryAddress":"fe80::7f1:49e7:768c:aa73",
      "status":"Master",
      "effectivePriority":110,
      "masterAdverInterval":1000,
      "skewTime":570,
      "masterDownInterval":3570,
      "stats":{
        "adverTx":13,
        "adverRx":5,
        "neighborAdverTx":1,
        "transitions":2
      },
      "addresses":[
        "2001:50::1"
      ]
    }
  }
]
```

Ticket:#3502432

Issue:3502432

Testing: UT done

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