Add bevy_fbx, an FBX loader based on ufbx

[VitalyAnkh]

Objective

• Support FBX asset loading via ufbx
• Fixes Support FBX file loading #15705

Solution

• Add bevy_fbx, an FBX loader based on ufbx.
• Why using ufbx? It is a thoroughly tested, single-file FBX loader that now powers Godot’s native FBX importer and Blender’s new official importer; it also ships with first-party Rust bindings.

Testing

• example load_fbx, asset_loading.

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