feat(healthcheck): combination of ICMP and TCP+TLS checks (#2923)

- New option: `HEALTH_ICMP_TARGET_IP` defaults to `0.0.0.0` meaning use the VPN server public IP address.
- Options removed: `HEALTH_VPN_INITIAL_DURATION` and `HEALTH_VPN_ADDITIONAL_DURATION` - times and retries are handpicked and hardcoded.
- Less aggressive checks and less false positive detection

internal/healthcheck/interfaces.go

@@ -0,0 +1,8 @@
+package healthcheck
+
+type Logger interface {
+	Debugf(format string, args ...any)
+	Info(s string)
+	Warnf(format string, args ...any)
+	Error(s string)
+}