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/handler.go

@@ -9,13 +9,15 @@ import (
 type handler struct {
 	healthErr   error
 	healthErrMu sync.RWMutex
+	logger      Logger
 }
 
 var errHealthcheckNotRunYet = errors.New("healthcheck did not run yet")
 
-func newHandler() *handler {
+func newHandler(logger Logger) *handler {
 	return &handler{
 		healthErr: errHealthcheckNotRunYet,
+		logger:    logger,
 	}
 }