Support FBX file loading

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

[FBX](https://en.wikipedia.org/wiki/FBX) is a commonly used file format for 3d models. Many 3d models & asset packs from asset stores, eg the popular [Unity AssetStore](https://assetstore.unity.com/), only supply models & materials in `.fbx` format.

By not supporting FBX, Bevy users must convert assets manually which is especially painful as some asset packs can have hundreds of model files. This hinders teams trying to leverage commercial assets to speed up their development.

## What solution would you like?

Support FBX file import through a loader/processor.

A loader like [bevy_mod_fbx](https://github.com/nicopap/bevy_mod_fbx) could be upstreamed.

Alternatively, [like Godot](https://github.com/godotengine/godot/pull/59653), leverage [fbx2gltf](https://github.com/godotengine/FBX2glTF) or [ufbx](https://github.com/ufbx/ufbx)(current Godot default) within the engine to perform the conversion seamlessly for users

## What alternative(s) have you considered?

Relying on 3rd party crates (none seem up to date), or manually converting via Blender.

Add fbx -> gltf conversion into the bevy cli tool.

## Additional context

I searched for "fbx" issues previously, but couldn't find one in the repo. I assume this has been thought of before, but I wanted to formalise it.

Start by comparing bevy_mod_fbx with the fbx2gltf and ufbx approaches described in the issue, including whether conversion belongs in the engine or Bevy CLI tool. Review the relevant upstream projects and define a supported FBX import path; done means Bevy users can load FBX assets without manual conversion.

Written by the indexing model from the issue text.