feat: FlutterGen plugin

[ronnnnn]

Summary

Add FlutterGen plugin.

Description:

• Tool repo URL: https://github.com/FlutterGen/flutter_gen
• Plugin repo URL: https://github.com/FlutterGen/asdf-fluttergen

Checklist

• [x] Format with scripts/format.bash
• [x] Test locally with scripts/test_plugin.bash --file plugins/<your_new_plugin_name>
• [x] Your plugin CI tests are green
  • Tip: use the plugin_test action from asdf-actions in your plugin CI