Fixes an issue when uninstalling the VPN over UDS, where the menu app…

… wouldn't quit (#866)

Task/Issue URL: https://app.asana.com/0/1207603085593419/1207659744300515/f

iOS PR: duckduckgo/iOS#2999
macOS PR: duckduckgo/macos-browser#2911

What kind of version bump will this require?: Major/Minor/Patch

## Description

Fixes stopping the VPN agent after uninstalling the VPN in App Store builds.

Sources/NetworkProtection/ExtensionMessage/ExtensionRequest.swift

@@ -25,6 +25,7 @@ public enum VPNCommand: Codable {
     case sendTestNotification
     case uninstallVPN
     case disableConnectOnDemandAndShutDown
+    case quitAgent
 }
 
 public enum ExtensionRequest: Codable {

Sources/NetworkProtection/PacketTunnelProvider.swift

@@ -1062,6 +1062,9 @@ open class PacketTunnelProvider: NEPacketTunnelProvider {
         case .uninstallVPN:
             // Since the VPN configuration is being removed we may as well reset all state
             handleResetAllState(completionHandler: completionHandler)
+        case .quitAgent:
+            // No-op since this is intended for the agent app
+            break
         }
     }