bgpd: Add JSON output for `show ip bgp peer-group` command

bgpd: Add JSON output for `show ip bgp peer-group` command

```
exit1-debian-9# show bgp peer-group

BGP peer-group test, remote AS 123
  Peer-group type is external
  Configured address-families: IPv4 Unicast; IPv6 Unicast;
  1 IPv4 listen range(s)
    192.168.100.0/24
  2 IPv6 listen range(s)
    2001:db8:1::/64
    2001:db8:2::/64
  Peer-group members:
    192.168.200.1  Active
    2001:db8::1  Active

BGP peer-group test3
  Peer-group type is external
  Configured address-families: IPv4 Unicast;
exit1-debian-9#
exit1-debian-9# show bgp peer-group json
{
  "test":{
    "remoteAs":123,
    "type":"external",
    "addressFamiliesConfigured":[
      "IPv4 Unicast",
      "IPv6 Unicast"
    ],
    "dynamicRanges":{
      "IPv4":{
        "count":1,
        "ranges":[
          "192.168.100.0\/24"
        ]
      },
      "IPv6":{
        "count":2,
        "ranges":[
          "2001:db8:1::\/64",
          "2001:db8:2::\/64"
        ]
      }
    },
    "members":{
      "192.168.200.1":{
        "status":"Active"
      },
      "2001:db8::1":{
        "status":"Active"
      }
    }
  },
  "test3":{
    "type":"external",
    "addressFamiliesConfigured":[
      "IPv4 Unicast"
    ]
  }
}
```

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>