Expose useArtboardSize through RiveAnimation

[neilresniksonos]

Right now, if someone wants to access the useArtboardSize parameter on a Rive widget, they must re-implement all the code in RiveAnimation which handles loading and initializing the animation. This would simplify things so that if all one wants to do on top of what Rive Animation is already doing is give the resulting widget an intrinsic size, they can, without rewriting a bunch of boilerplate code.