bgpd: Add CLI for overlay index recursive resolution
Gateway IP overlay index of the remote type-5 route is resolved
recursively using remote type-2 route. For the purpose of this
recursive resolution, for each L2VNI, we build a hash table of the
remote IP addresses received by remote type-2 routes.
For the topologies where overlay index resolution is not needed, we
do not need to build this remote-ip-hash.
Thus, make the recursive resolution of the overlay index conditional on
"enable-resolve-overlay-index" configuration.