Add `package` parameter to `RiveAnimation.asset()` for package support

Problem

RiveAnimation.asset() doesn't support a package parameter, preventing Rive animations from being stored in reusable packages shared across projects.

Proposed Solution

Add a package parameter to match Flutter's convention:

RiveAnimation.asset(
  'animations/my_animation.riv',
  package: 'my_package',
)

Context

This pattern is standard across Flutter's asset APIs:

// Image
Image.asset('path', package: 'my_package')

// SvgPicture
SvgPicture.asset('path', package: 'my_package')

Benefit

Enables sharing Rive animations across multiple projects via packages, improving code reusability.

Note

I know that you’re deprecating RiveAnimation.asset() in version 0.14. In v0.14, it will look something like this:

 late final fileLoader = FileLoader.fromAsset(
        'animations/my_animation.riv',
        riveFactory: Factory.rive,
        package: 'my_package'
   );

Start at the RiveAnimation.asset() API and compare its asset-loading behavior with the proposed FileLoader.fromAsset() call. Verify how Flutter asset packages are passed through, then confirm that an animation from a reusable package loads with the package argument; account for the stated deprecation of RiveAnimation.asset() in version 0.14.