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/vpn/interfaces.go

@@ -99,3 +99,13 @@ type CmdStarter interface {
 		stdoutLines, stderrLines <-chan string,
 		waitError <-chan error, startErr error)
 }
+
+type HealthChecker interface {
+	SetConfig(tlsDialAddr string, icmpTarget netip.Addr)
+	Start(ctx context.Context) (runError <-chan error, err error)
+	Stop() error
+}
+
+type HealthServer interface {
+	SetError(err error)
+}