fpm: Add SRv6 VPN SIDs to `fpm.proto`

fpm: Add SRv6 VPN SIDs to `fpm.proto`

Add two optional fields to the AddRoute protobuf message to support
SRv6 VPN use cases:
* `srv6_vpn_sid`: used to associate an SRv6 VPN SID to a route; if
present, the router will steer the traffic that matches the prefix by
encapsulating the payload in an outer IPv6 header where the destination
address is the SRv6 VPN SID provided
* `srv6_encap_source_address`: source Address of outer encapsulating
IPv6 header

This is a preliminary commit to support sending SRv6 Local SIDs and VPN
SIDs via protobuf.

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