bgpd: Show Long-lived Graceful Restart timer remaining per prefix

bgpd: Show Long-lived Graceful Restart timer remaining per prefix

```
exit1-debian-11# sh ip bgp 100.100.100.100/32
BGP routing table entry for 100.100.100.100/32, version 7
Paths: (2 available, best #2, table default)
  Advertised to non peer-group peers:
  home-spine1.donatas.net(192.168.0.2)
  65002, (stale)
    192.168.10.17 from donatas-pc(192.168.10.17) (0.0.0.0)
      Origin incomplete, valid, external
      Community: llgr-stale
      Last update: Thu Jan 13 08:58:08 2022
      Time until Long-lived stale route deleted: 18
  65001
    192.168.0.2 from home-spine1.donatas.net(192.168.0.2) (2.2.2.2)
      Origin incomplete, metric 0, valid, external, best (First path received)
      Last update: Thu Jan 13 08:57:56 2022
```

```
~# vtysh -c 'show ip bgp 100.100.100.100/32 json' | jq '."paths"[] | ."llgrSecondsRemaining"'
17
```

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