isisd: add json support to display spf paths in 'show isis route'

isisd: add json support to display spf paths in 'show isis route'

The 'show isis route json' command never displays the list of
paths. Add the json support for this sub-part.

> # show isis route json
> [..]
> "ipv6-paths":[
>  {
>   "Vertex":"rt1",
>   "Type":"",
>   "Metric":0,
>   "Next-Hop":"",
>   "Interface":"",
>   "Parent":""
>  },
>  {
>    "Vertex":"2001:db8:1000::1\/128",
>    "Type":"IP6 internal",
>    "Metric":0,
>    "Next-Hop":"",
>    "Interface":"",
>    "Parent":"rt1(4)"
>  },

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