Add feature flag for SKAN API (#993)

Task/Issue URL:  https://app.asana.com/0/0/1208294274484279/f
iOS PR:  duckduckgo/iOS#3356
macOS PR: duckduckgo/macos-browser#3265
What kind of version bump will this require?: Minor

**Description**:
Add feature flag for SKAN API

Sources/BrowserServicesKit/PrivacyConfig/Features/PrivacyFeature.swift

@@ -56,6 +56,7 @@ public enum PrivacyFeature: String {
     case newTabPageImprovements
     case syncPromotion
     case autofillSurveys
+    case marketplaceAdPostback
 }
 
 /// An abstraction to be implemented by any "subfeature" of a given `PrivacyConfiguration` feature.