bgpd: implement "no sid vpn export auto" per AF in vtysh

bgpd: implement "no sid vpn export auto" per AF in vtysh

An example of implemented command for IPv4 AF is presented below.

dev-vm(config-router-af)# do show bgp segment-routing srv6
locator_name: Locator1
locator_chunks:
- 2001:db8:2:5::/64
  block-length: 40
  node-length: 24
  func-length: 16
  arg-length: 0
functions:
bgps:
- name: default
  vpn_policy[AFI_IP].tovpn_sid: (null)
  vpn_policy[AFI_IP6].tovpn_sid: (null)
  per-vrf tovpn_sid: (null)
- name: red
  vpn_policy[AFI_IP].tovpn_sid: (null)
  vpn_policy[AFI_IP6].tovpn_sid: (null)
  per-vrf tovpn_sid: (null)
dev-vm(config-router-af)# sid vpn export auto
^^^<<<<<<<<<<<<<<<<<<<<<< 1. configure sid per af
dev-vm(config-router-af)# do show bgp segment-routing srv6
locator_name: Locator1
locator_chunks:
- 2001:db8:2:5::/64
  block-length: 40
  node-length: 24
  func-length: 16
  arg-length: 0
functions:
- sid: 2001:db8:2:5:1::
  locator: Locator1
bgps:
- name: default
  vpn_policy[AFI_IP].tovpn_sid: (null)
  vpn_policy[AFI_IP6].tovpn_sid: (null)
  per-vrf tovpn_sid: (null)
- name: red
  vpn_policy[AFI_IP].tovpn_sid: 2001:db8:2:5:1::
^^^<<<<<<<<<<<<<<<<<<<<< 2. sid is configured
  vpn_policy[AFI_IP6].tovpn_sid: (null)
  per-vrf tovpn_sid: (null)
dev-vm(config-router-af)# no sid vpn export auto
^^^<<<<<<<<<<<<<<<<<<<< 3. deconfigure sid per af
dev-vm(config-router-af)# do show bgp segment-routing srv6
locator_name: Locator1
locator_chunks:
- 2001:db8:2:5::/64
  block-length: 40
  node-length: 24
  func-length: 16
  arg-length: 0
functions:
bgps:
- name: default
  vpn_policy[AFI_IP].tovpn_sid: (null)
  vpn_policy[AFI_IP6].tovpn_sid: (null)
  per-vrf tovpn_sid: (null)
- name: red
  vpn_policy[AFI_IP].tovpn_sid: (null)
^^^<<<<<<<<<<<<<<<<<<< 4. sid is deconfigured
  vpn_policy[AFI_IP6].tovpn_sid: (null)
  per-vrf tovpn_sid: (null)

Signed-off-by: Dmytro Shytyi <dmytro.shytyi@6wind.com>