pim6d: Modify "show ipv6 mld join json" o/p

pim6d: Modify "show ipv6 mld join json" o/p

Currently "show ipv6 mld join json" o/p is
frr# show ipv6 mld joins json
{
  "default":{
    "ens192":{
      "ff02:2":{
        "::":{
          "state":"JOIN",
          "created":"00:01:50.595",
          "lastSeen":"00:00:38.403",
        }
      }
    }
  }
}

Here, I modified the o/p as below for better understanding.
frr# show ipv6 mld joins json
{
  "default":{
    "vrf":"default",
    "ens192":{
      "ff02::2":{
        "*":{
          "state":"JOIN",
          "created":"00:00:42.766",
          "lastSeen":"00:00:05.266"
        }
      }
    }
  }
}

Issue: #12755

Signed-off-by: Sarita Patra <saritap@vmware.com>