feat(windows): add post-package command option for Windows packaging

[annd22]

Add --post-package-windows-cmd option to execute custom commands between build and package steps for Windows platform. This enables custom file operations (like code signing) on the built executable before packaging.

Changes:

• Add new CLI option --post-package-windows-cmd in command_package.dart
• Implement command execution logic in unified_distributor.dart
• Add proper error handling and logging for custom command execution

Example usage: fastforge package --platform windows --targets exe --post-package-windows-cmd="call ./sign-file.bat"

[annd22]

This pull request is for: https://github.com/fastforgedev/fastforge/issues/272