feat(healthcheck): HEALTH_ICMP_TARGET_IP -> HEALTH_ICMP_TARGET_IPS

- Specify fallback ICMP IP addresses
- Defaults changed from 1.1.1.1 to 1.1.1.1,8.8.8.8
- Small periodic check cycles through addresses as it fails and moves to retry

internal/vpn/interfaces.go

@@ -101,7 +101,7 @@ type CmdStarter interface {
 }
 
 type HealthChecker interface {
-	SetConfig(tlsDialAddrs []string, icmpTarget netip.Addr)
+	SetConfig(tlsDialAddrs []string, icmpTargetIPs []netip.Addr)
 	Start(ctx context.Context) (runError <-chan error, err error)
 	Stop() error
 }