bgpd: cli for SRv6 SID alloc to redirect to vrf (step4)

bgpd: cli for SRv6 SID alloc to redirect to vrf (step4)

This commit add cil to configure BGP SRv6-VPN sid allocation.
Almost mechanism are based on BGP MPLS-VPN.

User can allocate and export sid with using following config.
Then bgpd try to allocate new SID to redirect vpn to vrf using
SRv6 localsid End.DT4/DT6. Currently linux kernel will regect
End.DT4 route install due to no-implementation.
(at-least today's FRR's ci kernel.)

So now we only supports BGP SRv6-VPNv6.

router bgp 1
 segment-routing srv6
  locator loc1
 !
 address-family ipv6 vpn
 exit-address-family
!
router bgp 1 vrf vrf10
 address-family ipv6 unicast
  sid vpn export 1    !!(option1)!!
  sid vpn export auto !!(option2)!!
 exit-address-family
!

Signed-off-by: Hiroki Shirokura <slank.dev@gmail.com>