[UX2.0][20.15] add support for vrrp preempt delay

[ctvedt]

Support for configuring VRRP minimum preempt delay was added to configuration groups in 20.15 (GUI). Would like to see this implemented for the provider as well! (IPv4 and IPv6)

GUI (IPv4):

Resources:

• sdwan_service_lan_vpn_interface_ethernet_feature