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>

README.md

@@ -1,18 +1,31 @@
 # flutter_usb_serial
 
-A new Flutter plugin project.
+Flutter USB serial transport for Android and DriverKit-enabled iPadOS.
 
-## Getting Started
+## Platforms
 
-This project is a starting point for a Flutter
-[plug-in package](https://flutter.dev/to/develop-plugins),
-a specialized package that includes platform-specific implementation code for
-Android and/or iOS.
+- Android uses `usb-serial-for-android` and the normal Android USB permission
+  flow.
+- iPadOS 16+ uses Swift Package Manager and a user client exposed by a
+  USBDriverKit extension embedded in the host app. DriverKit USB support is
+  available only on iPads with an M-series chip.
 
-For help getting started with Flutter development, view the
-[online documentation](https://docs.flutter.dev), which offers tutorials,
-samples, guidance on mobile development, and a full API reference.
+The package contains the SwiftPM client plugin, not a reusable pre-signed
+DriverKit extension. Apple provisions USB transport entitlements to the host
+app and driver for a specific USB vendor. The host app must:
 
-The plugin project was generated without specifying the `--platforms` flag, no platforms are currently supported.
-To add platforms, run `flutter create -t plugin --platforms <platforms> .` in this directory.
-You can also find a detailed instruction on how to add platforms in the `pubspec.yaml` at https://flutter.dev/to/pubspec-plugin-platforms.
+1. Embed a DriverKit extension whose `IOUserClass` is
+   `DaliMasterEspressifUsbDriver`, or adapt the client service name.
+2. Add `com.apple.developer.driverkit.communicates-with-drivers` to the app.
+3. Sign the extension with `com.apple.developer.driverkit` and a
+   `com.apple.developer.driverkit.transport.usb` entitlement approved for its
+   USB vendor ID.
+4. Enable Swift Package Manager integration for Flutter iOS plugins.
+
+Call `FlutterUsbSerial.isDriverAvailable()` during startup before showing a USB
+transport. It returns `true` only when a compatible driver service is loaded.
+
+The DaliMaster host implementation is deliberately restricted to Espressif
+vendor ID `0x303A` and the USB Serial/JTAG interface. Serial parameter and modem
+line methods are accepted for API compatibility; the ESP USB Serial/JTAG bulk
+transport does not use UART baud or line coding.