bgpd: add 'show bgp label-nexthop [detail]' command

bgpd: add 'show bgp label-nexthop [detail]' command

The following command is made available to list the labels
allocated per-nexthop, along with the paths registered to it.

 > # show bgp vrf vrf1 label-nexthop
 > Current BGP label nexthop cache for IP, VRF vrf1
 >  192.0.2.11, label 20 #paths 3
 >    if r1-eth1
 >    Last update: Mon Jan 16 18:52:11 2023
 >  192.0.2.12, label 17 #paths 2
 >    if r1-eth1
 >    Last update: Mon Jan 16 18:52:08 2023
 >  192.0.2.14, label 18 #paths 1
 >    if r1-eth1
 >    Last update: Mon Jan 16 18:52:07 2023
 >  192.168.255.13, label 19 #paths 1
 >    if r1-eth2
 >    Last update: Mon Jan 16 18:52:10 2023

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