hi team

the idea this PR , is to give the ability to transport labels over a next-hop learned in a GRE tunnel, the path chosen by the next-hop uses a GRE interface, and there is a route-map configured at inbound side of ipv4-vpn or ipv6-vpn address family with following syntax:

FRR:

route-map l3vpn permit 10
 set l3vpn next-hop encapsulation gre
exit
!