Add optional package parameter to RiveAnimation.asset constructor

[tozu]

The RiveAnimation.asset constructor assumes that all animations are located in the Flutter app's assets-folder.

It does not provide an way to specify an optional package name that holds and provides animations in its assets-folder.

With this PR the RiveAnimation.asset constructor usage would be analog to Flutter's Image.asset constructor.

The implementation of adding the optional package-path is accomplished in similar fashion (see Image.asset source-code here)