Replace FBX C++/CLI importer with a cross-platform FBX library

[JeromyWalsh]

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[sandsalamand]

I think Stride should copy Godot's model of encouraging glTF and using FBX2glTF to automatically convert imported FBX files.

[sandsalamand]

What is the state of glTF support in Stride? Would this be an option?

[SeleDreams]

gltf is already supported from what I know.
Also I had the project to move the fbx importer to uFBX but I was waiting for the transition to the new c++ build system to work on it

[sandsalamand]

Based on #1030, it seems that glTF is not fully supported yet.

@SeleDreams Is there a reason you would prefer to maintain independent FBX importer support, instead of relying on FBX2glTF?

[gamingrobot]

Since we now use Silk.NET.Assimp #1719 do we still need Stride.Importer.FBX?