同时加载多个广告

func prepare(_ format: AdFormat) async -> VideoAd? {
    do {
        let ad = try await ads.format(format)
        try await ad.load()
        ad.delegate = self
        return ad
    } catch {
        return nil                     // 这次该格式无填充
    }
}

async let overlay = prepare(.overlay)
async let pause = prepare(.pause)
async let linear = prepare(.linear)
(overlayAd, pauseAd, linearAd) = await (overlay, pause, linear)