lib: add flavors display in json attributes

lib: add flavors display in json attributes

Add flavors display in json nexthop attributes.

> # ip -6 route add 2007::1 encap seg6local action End flavors psp dev dum1
> # vtysh -c "show ipv6 route json"
> [..]
> ,"2007::1/128":[{"prefix":"2007::1/128","prefixLen":128,"protocol":"kernel","vrfId":0,
> "vrfName":"default","selected":true,"destSelected":true,"distance":0,"metric":1024,
> "installed":true,"table":254,"internalStatus":16,"internalFlags":8,"internalNextHopNum":1,
> "internalNextHopActiveNum":1,"nexthopGroupId":28,"installedNexthopGroupId":28,"uptime":"00:04:20",
> "nexthops":[{"flags":3,"fib":true,"directlyConnected":true,"interfaceIndex":19,"interfaceName":"dum1",
> "active":true,"weight":1,"seg6local":{"action":"End","sidStructure":{"blockLen":0,"nodeLen":0,"funcLen":0,"argLen":0}},
> "seg6localContext":{"flavors":["psp"]}}]}]
> [..]

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