lib: Extend nexthop vty helper to display SRv6 encap behavior

lib: Extend nexthop vty helper to display SRv6 encap behavior

This commit extends the `show ip route` and `show ipv6 route`
output to display the SRv6 encapsulation behavior associated with the
SRv6 nexthop.

```
router# show ipv6 route
Codes: K - kernel route, C - connected, L - local, S - static,
       R - RIPng, O - OSPFv3, I - IS-IS, B - BGP, N - NHRP,
       T - Table, v - VNC, V - VNC-Direct, A - Babel, D - SHARP,
       F - PBR, f - OpenFabric, t - Table-Direct,
       > - selected route, * - FIB route, q - queued, r - rejected, b - backup
       t - trapped, o - offload failure

IPv6 unicast VRF default:
S>* 2001:db8:1:1::2/128 [1/0] is directly connected, sr0, seg6 fcbb:bbbb:7:8:fe00::, weight 1, 00:05:49
S>* 2001:db8:1:1::3/128 [1/0] is directly connected, sr0, seg6 fcbb:bbbb:7:8:fe00::, encap behavior H.Encaps.Red, weight 1, 00:05:49
...

```

Signed-off-by: Carmine Scarpitta <cscarpit@cisco.com>