Integrate `PShape` and `PImage` with the Bevy asset system

We want users to be able to load things like gltfs to enable interoperability with tools like Blender. As such, we'll need to create WebGPU flavors for both PShape and PImage that are backed by a Bevy Mesh and Image in the Bevy asset system. We can take advantage of Bevy's asset loading to do so. One tricky bit will be that Bevy's asset loading is async, so we may need to block and poll until it's complete or alternatively upstream a blocking API to make our life easier.

Start by tracing the PShape and PImage entry points and how their WebGPU flavors represent assets. Then examine Bevy's Mesh and Image asset loading, including the asynchronous behavior described in the issue. Done means both types are backed by Bevy assets and can load formats such as gltfs, with the blocking or polling approach resolved.