zebra: add json support to 'show debugging label-table'

zebra: add json support to 'show debugging label-table'

Add the json keyword to dump the label chunks of
the zebra label manager in json format.

>dut# show debugging label-table json
> {
>   "chunks":[
>     {
>       "protocol":"bgp",
>       "instance":0,
>       "sessionId":1,
>       "start":16,
>       "end":16,
>       "dynamic":true
>     },
>     {
>       "protocol":"ldp",
>       "instance":0,
>       "sessionId":1,
>       "start":17,
>       "end":80,
>       "dynamic":true
>     }
>   ]
> }

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