Backup liveness detection on EVPN dual homed peers

We've added support for backup liveness detection for EVPN peers. This feature addresses a gap in the core isolation feature that halts traffic within a data center with two spine devices when the BGP session between those devices goes down. You can configure backup liveness detection to track the state of the adjacent peer in conjunction with core isolation to ensure that the links to one of the spine devices stay up even during a BGP session failure. This configuration allows traffic within the data center to continue