Asset `Process` should use a save path instead of a AsyncWrite

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

Bevy's asset processor integrates its loaders and savers, needs users to write transformed asset to a AsyncWrite, but what if I can only write transformed asset to a path?
Use cases: 1. Invoke blender to transform .blend file to .gltf file. 2. Invoke basisu tool to compress textures, where we don't load and save assets using bevy at all, we just need to know source path and destination path.

See also Godot's resource importer https://docs.godotengine.org/en/latest/classes/class_editorimportplugin.html#class-editorimportplugin

## What solution would you like?

Asset processor should just require source path and destination path instead of requiring a writer.

## What alternative(s) have you considered?

Don't use bevy's processor and write my own scripts or Rust application to transform assets.