Prepare system extension support

SystemExtension/PacketTunnelProvider.swift

@@ -0,0 +1,15 @@
+import Library
+import NetworkExtension
+
+class PacketTunnelProvider: ExtensionProvider {
+    override func startTunnel(options: [String: NSObject]?) async throws {
+        guard let usernameObject = options?["username"] else {
+            writeFatalError("missing start options")
+            return
+        }
+        let username = usernameObject as! NSString
+        FilePath.sharedDirectory = URL(filePath: "/Users/\(username)/Library/Group Containers/\(FilePath.groupName)")
+        self.username = String(username)
+        try await super.startTunnel(options: options)
+    }
+}