[preview]Add PIC support in the srv6 VPN scenario.

[zice312963205]

What I did [HLD] BGP PIC HLD

Why I did it This PR implements the separation of nh_srv6-related information, which was originally stored in the nexthop, in the srv6-vpn scenario. It generates a new pic nexthop that contains only the next-hop forwarding information. Meanwhile, the original nexthop containing nh_srv6 is used as pic context information and indexed separately. This is used to achieve fast switching of nexthop group members by identifying the corresponding members in the nexthop group based on route changes, and then performing operations on the members.

This PR is temporarily for review purposes. Documentation and test cases will be added later, and a complete PR will be resubmitted.