Update for Unity 2023 support and migrate to WebViewManager system

[t-34400]

Summary

This pull request includes major updates to support Unity 2023+, improve compatibility, and enhance development usability.

What's Changed

✅ Functionality

• Added support for Unity 2023.1 and newer
  • Adapted to breaking changes in UnityPlayer behavior
  • Switched to GameActivity-based launch configuration
• Removed dependency on subclassing UnityPlayerActivity / UnityPlayerGameActivity
  • Improved compatibility with various Unity runtime environments
• Bundled all Android-side code into an .aar
  • Simplified integration and dependency management

✅ API Changes

• Introduced new WebViewManager component
  • Replaces WebViewBitmapReceiver and WebViewControllerClient
  • Centralizes all WebView operations (loading, JS evaluation, keyboard input, etc.)
  • Exposes new UnityEvents:
    • UnityEvent<string> urlChanged
    • UnityEvent<ReceivedData> dataReceived
• Deprecated StartUpdate() / StopUpdate() methods
  • Texture updates are now controlled via enabled flag on WebViewManager

✅ Documentation

• Rewrote README.md:
  • Added “What's New” section for v2.0.0
  • Split installation instructions for Unity 2022 and 2023
  • Updated usage guide to reflect new WebViewManager design
  • Included sample code and configuration examples
  • Added link to AAR source code

Breaking Changes

• Projects using WebViewBitmapReceiver or WebViewControllerClient must migrate to WebViewManager
• Manifest and Gradle template setup differs between Unity versions
• Event listeners now receive ReceivedData instead of two strings