bgpd: add 'show bgp mplsvpn-nh-label-bind' command

bgpd: add 'show bgp mplsvpn-nh-label-bind' command

There is no 'show command' to use for troubleshooting
purposes.
Add a new show command to dump the cache entry of the
MPLS VPN nexthop label bind cache table.
> show bgp [vrf NAME] mplsvpn-nh-label-bind [detail]

The below command illustrates its output:
> dut# show bgp mplsvpn-nh-label-bind  detail
> Current BGP mpls-vpn nexthop label bind cache, VRF default
>  192.168.1.3, label 102, local label 18 #paths 3
>   interface r2-eth1
>   Last update: Mon May 22 14:39:42 2023
>   Paths:
>     1/3 172.31.3.0/24 VRF default flags 0x418
>     1/3 172.31.2.0/24 VRF default flags 0x418
>     1/3 172.31.1.0/24 VRF default flags 0x418
>  192.0.2.1, label 101, local label 19 #paths 1
>   interface r2-eth0
>   Last update: Mon May 22 14:39:43 2023
>   Paths:
>     1/3 172.31.0.0/24 VRF default flags 0x418

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