bgpd: add json support for BGP L3NHG values

bgpd: add json support for BGP L3NHG values

Some json attributes are missing for L3NHG values.

> PE1# show bgp vrf all detail
> [..]
> Instance vrf-purple:
> BGP table version is 1, local router ID is 27.3.0.85, vrf id 7
> Default local pref 100, local AS 65000
> BGP routing table entry for fe80::4620:ff:feff:ff01/128, version 1
> Paths: (1 available, best #1, vrf vrf-purple)
>   Not advertised to any peer
>   Imported from 10.30.30.30:5:[2]:[0]:[48]:[44:20:00:ff:ff:01]:[128]:[fe80::4620:ff:feff:ff01], VNI 4000  Local
>     ::ffff:a1e:1e1e (metric 20) from 10.30.30.30 (10.30.30.30) announce-nh-self
>       Origin IGP, localpref 100, valid, internal, best (First path received)
>       Extended Community: RT:65000:4000 ET:8
>       Last update: Thu Aug 22 18:23:38 2024
>
> Displayed 1 routes and 1 total paths

> PE1# show bgp vrf all json detail
> {
> "vrf-purple":{
>  "vrfId": 7,
>  "vrfName": "vrf-purple",
>  "tableVersion": 1,
>  "routerId": "27.3.0.85",
>  "defaultLocPrf": 100,
>  "localAS": 65000,
>  "routes": { "fe80::4620:ff:feff:ff01/128": [{"importedFrom":"10.30.30.30:5","l3nhg":false,"l3nhgActive":false, "vni": "4000",
> "aspath":{"string":"Local","segments":[],"length":0},"announceNexthopSelf":true,"origin":"IGP","locPrf":100,
> "valid":true,"version":1,"bestpath":{"overall":true,"selectionReason":"First path received"},
> "extendedCommunity":{"string":"RT:65000:4000 ET:8"},"lastUpdate":{"epoch":1724343817,
> "string":"Thu Aug 22 18:23:37 2024\n"},
> "nexthops":[{"ip":"::ffff:a1e:1e1e","hostname":"PE2","afi":"ipv6",
> "scope":"global","metric":20,"accessible":true,"used":true}],
> "peer":{"peerId":"10.30.30.30","routerId":"10.30.30.30","hostname":"PE2","type":"internal"}}]
>  }  }
> }

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>