Method ShowInterstitialAd
ShowInterstitialAd(Action<bool, int>)
Shows the highest-eCPM ready interstitial unit. Respects the InterstitialInterval
cooldown between consecutive shows. The callback receives (true, durationSeconds) on close,
or (false, 0) when not shown due to cooldown, no ready unit, or a display failure.
public static void ShowInterstitialAd(Action<bool, int> onClosed = null)