Enhance USB serial plugin for Android and iPadOS with DriverKit support

- Updated README.md to reflect platform support for iPadOS.
- Implemented isDriverAvailable method in FlutterUsbSerialPlugin for DriverKit readiness check.
- Added DriverKitSerialClient C source and header files for iPadOS integration.
- Created Swift Package for iPadOS support.
- Updated pubspec.yaml and flutter_usb_serial.dart to clarify platform compatibility.
- Enhanced tests to verify isDriverAvailable functionality.

Signed-off-by: Tony <tonylu@tony-cloud.com>

test/flutter_usb_serial_test.dart

@@ -1,10 +1,16 @@
+import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter_usb_serial/flutter_usb_serial.dart';
 import 'package:flutter_usb_serial/flutter_usb_serial_method_channel.dart';
 
 void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
   test('$MethodChannelFlutterUsbSerial can be constructed', () {
-    expect(MethodChannelFlutterUsbSerial(), isA<MethodChannelFlutterUsbSerial>());
+    expect(
+      MethodChannelFlutterUsbSerial(),
+      isA<MethodChannelFlutterUsbSerial>(),
+    );
   });
 
   test('UsbSerialDevice maps fields from method channel payloads', () {
@@ -25,4 +31,22 @@ void main() {
     expect(device.serialNumber, 'SN-42');
     expect(device.toMap()['deviceId'], 7);
   });
+
+  test(
+    'isDriverAvailable forwards the native DriverKit readiness result',
+    () async {
+      const channel = MethodChannel('flutter_usb_serial/methods');
+      TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+          .setMockMethodCallHandler(channel, (call) async {
+            expect(call.method, 'isDriverAvailable');
+            return true;
+          });
+      addTearDown(() {
+        TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+            .setMockMethodCallHandler(channel, null);
+      });
+
+      expect(await FlutterUsbSerial.isDriverAvailable(), isTrue);
+    },
+  );
 }