Add AI Chat feature flag (#1031)

Task/Issue URL:
https://app.asana.com/0/1204167627774280/1208539744359975/f
iOS PR: duckduckgo/iOS#3468
macOS PR: duckduckgo/macos-browser#3439
What kind of version bump will this require?: Minor

**Description**:
Add feature flag for AI Chat settings

Sources/BrowserServicesKit/PrivacyConfig/Features/PrivacyFeature.swift

@@ -60,6 +60,7 @@ public enum PrivacyFeature: String {
     case marketplaceAdPostback
     case autocompleteTabs
     case networkProtection
+    case aiChat
 }
 
 /// An abstraction to be implemented by any "subfeature" of a given `PrivacyConfiguration` feature.
@@ -97,6 +98,18 @@ public enum DBPSubfeature: String, Equatable, PrivacySubfeature {
     case freemium
 }
 
+public enum AIChatSubfeature: String, Equatable, PrivacySubfeature {
+    public var parent: PrivacyFeature {
+        .aiChat
+    }
+
+    /// Displays the settings item for showing a shortcut in the Application Menu
+    case applicationMenuShortcut
+
+    /// Displays the settings item for showing a shortcut in the Toolbar
+    case toolbarShortcut
+}
+
 public enum NetworkProtectionSubfeature: String, Equatable, PrivacySubfeature {
     public var parent: PrivacyFeature {
         .networkProtection