bgpd: add support for per-VRF SRv6 SID

bgpd: add support for per-VRF SRv6 SID

In the current implementation of bgpd, SRv6 SIDs can be configured only
under the address-family. This enables bgpd to leak IPv6 routes using
an SRv6 End.DT6 behavior and IPv4 routes using an SRv6 End.DT4
behavior. It is not possible to leak both IPv6 and IPv4 routes using a
single SRv6 SID.

This commit adds a new CLI command
"sid vpn per-vrf export <sid_idx|auto>" that enables bgpd to leak both
IPv6 and IPv4 routes using a single SRv6 SID (End.DT46 behavior).

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