Allow Using URLs for Image Files

**Description:**

Currently, Flutter Launcher Icons requires image files to be local assets, which can be limiting in some scenarios. This PR aims to enhance the package's functionality by allowing developers to specify image URLs instead of local file paths, thus providing more flexibility in how app launcher icons are generated.

**Changes Made:**

- Modified the `flutter_launcher_icons` package to accept image URLs as an alternative to local file paths when generating app launcher icons.

**Usage Example:**

```yaml
flutter_icons:
android: true
ios: true
image_url: https://example.com/app_icon.png
```

**Benefits:**

- Allows developers to fetch app icon images dynamically from remote sources, facilitating continuous integration and deployment workflows.
- Eliminates the need to store and manage image files locally, reducing the project's file size and simplifying asset management.