自iOS15之后，install postback将会发送一份给Ad Network（Vungle，Admob，Mopub等），一份发给广告主（或第三方Appsflyer，Singular，Branch）。
Starting in iOS 15, developers of advertised apps can opt-in to get copies of the winning postbacks that represent successful ad conversions for their app. To opt-in, configure a postback URL in your app. For more information see Configuring an Advertised App.
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Reference:
https://developer.apple.com/documentation/storekit/skadnetwork/receiving_ad_attributions_and_postbacks
https://developer.apple.com/documentation/storekit/skadnetwork
https://www.singular.net/blog/major-ios-15-attribution-news-advertiser-access-to-skadnetwork-postbacks-app-privacy-report-and-more-from-wwdc/
