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.

首先跟踪 PShape 和 PImage 的入口点，以及它们的 WebGPU 变体如何表示资产。然后检查 Bevy 的 Mesh 和 Image 资产加载，包括 issue 中描述的异步行为。当这两种类型都由 Bevy 资产支持、能够加载 gltfs 等格式，并且阻塞或轮询方式得到解决时，即可视为完成。