bgpd: show bgp vrfs per vrf level json support

bgpd: show bgp vrfs per vrf level json support

json support extended for show [ip] bgp vrfs <vrf-name> json

Before:
```
tor-2# show ip bgp vrfs default json
% JSON option not yet supported for specific VRF
tor-2#
tor-2# show bgp vrfs sym_1 json
% JSON option not yet supported for specific VRF
tor-2#
```

After:
```
tor-1# show ip bgp vrfs default json
{
  "default":{
    "type":"DFLT",
    "vrfId":0,
    "routerId":"27.0.0.7",
    "numConfiguredPeers":2,
    "numEstablishedPeers":2,
    "l3vni":0,
    "rmac":"00:00:00:00:00:00",
    "interface":"unknown"
  }
}
tor-1#
tor-1# show bgp vrfs sym_1 json
{
  "sym_1":{
    "type":"VRF",
    "vrfId":21,
    "routerId":"27.0.0.81",
    "numConfiguredPeers":0,
    "numEstablishedPeers":0,
    "l3vni":8888,
    "rmac":"44:38:39:ff:ff:25",
    "interface":"vlan490_l3"
  }
}
tor-1#

tor-1# show bgp vrfs test json
{
}
tor-1#
```

Ticket:#3314672

Issue:3314672

Testing: UT done

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