bgpd: add SRv6 per-VRF SID to SRv6 details

bgpd: add SRv6 per-VRF SID to SRv6 details

This commit adds the SRv6 per-VRF SID to the output of "show bgp segment-routing srv6".

Output example:

[...]
bgps:
- name: default
  vpn_policy[AFI_IP].tovpn_sid: none
  vpn_policy[AFI_IP6].tovpn_sid: none
  per-vrf tovpn_sid: none
- name: Vrf10
  vpn_policy[AFI_IP].tovpn_sid: none
  vpn_policy[AFI_IP6].tovpn_sid: none
  per-vrf tovpn_sid: 2001:db8:1:1:100::
[...]

Signed-off-by: Carmine Scarpitta <carmine.scarpitta@uniroma2.it>