Support alternate backends (like Unity, Unreal, Godot or Itch) to complement Bevy Assets / Marketplace

## What problem does this solve or what need does it fill?

Unity and Unreal have huge asset stores that many people have lots of assets in. It would make it easier to transition to using Bevy if Bevy was able to load in aasets from these stores. Especially things like 3D files, shaders, textures and audio.

## What solution would you like?
Crates for downloading packages from Unitys and Unreals asset stores.

## What alternative(s) have you considered?
Manual download via e.g. the Unity editor and then moving the files over into Bevy. But that requires having Unity installed and moving the files over manually.

Start by reviewing the Bevy Assets / Marketplace context and the proposed Unity and Unreal asset-store integrations. Define which stores and package types are in scope, including 3D files, shaders, textures, and audio; done would require an agreed design and crates for downloading the supported packages.

Written by the indexing model from the issue text.