Add feature flag (#1154)

**Required**:

Task/Issue URL:
https://app.asana.com/0/1204167627774280/1209108545833453/f
iOS PR: duckduckgo/iOS#3786
macOS PR: duckduckgo/macos-browser#3720
What kind of version bump will this require?: Minor

**Optional**:

Tech Design URL:
CC:

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

Sources/BrowserServicesKit/PrivacyConfig/Features/PrivacyFeature.swift

@@ -121,6 +121,9 @@ public enum AIChatSubfeature: String, Equatable, PrivacySubfeature {
 
     /// Displays the AI Chat icon in the iOS address bar while on a SERP.
     case addressBarShortcut
+
+    /// Web and native integration for opening AI Chat in a custom webview.
+    case deepLink
 }
 
 public enum NetworkProtectionSubfeature: String, Equatable, PrivacySubfeature {