bgpd: Add CLI for overlay index recursive resolution

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.

router bgp 65001
 bgp router-id 192.168.100.1
 neighbor 10.0.1.2 remote-as 65002
!
 address-family l2vpn evpn
  neighbor 10.0.1.2 activate
  advertise-all-vni
  enable-resolve-overlay-index----------> New configuration
 exit-address-family

Gateway IP overlay index will be resolved only if this configuration is present.

Signed-off-by: Ameya Dharkar <adharkar@vmware.com>