bgpd: Add "set evpn gateway-ip" clause for route-map

bgpd: Add "set evpn gateway-ip" clause for route-map

- Add following set clause for route-maps
  "set evpn gateway-ip <ipv4|ipv6 >A.B.C.D|X:X::X:X"
- When this route-map is applied as outboubd policy in BGP, it will set the
  gateway-ip in BGP attribute For EVPN type-5 routes.

Example configuration:

route-map RMAP-EVPN_GWIP permit 5
 set evpn gateway-ip ipv4 50.0.2.12
 set evpn gateway-ip ipv6 50:0:2::12

router bgp 101
 bgp router-id 10.100.0.1
 neighbor 10.0.1.2 remote-as 102
 !
 address-family l2vpn evpn
  neighbor 10.0.1.2 activate
  neighbor 10.0.1.2 route-map RMAP-EVPN_GWIP out
  advertise-all-vni
 exit-address-family

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