Assimp 4.1.0 update and glTF support

[tebjan]

Description

• updated assimp native references to official 4.1.0 binaries which supports glTF out of the box: https://github.com/assimp/assimp/releases/tag/v4.1.0
• unified place for supported file extensions
• added glTF and STL to supported formats

Related Issue

glTF importer: #96
the "Nothing to Import" issue is still present and not in scope of this PR: #440 and #336

Motivation and Context

loading models in new glTF and STL formats

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My change requires a change to the documentation.
• I have added tests to cover my changes.
• All new and existing tests passed.

[profan]

Does this handle the PBR material stuff in gltf and set that up on the xenko end at all or is that part of assimps interface and already magically works since xenko had hooked into it earlier? 🤔

I'd test it and see (but I won't be near a computer for a week) but I'm mostly curious as that's one of the biggest advantages of gltf, if not we can always make it the next step 👀👍

Nice either way!

[xen2]

@profan Nothing changed for materials, some additional work needed in this area.
@tebjan Thanks! Merged, along with some additional testing/error reporting. User should now have a much better idea has an idea of what went wrong rather than cryptic NullReferenceException as reported in #440 and #336.

[tebjan]

good to hear that errors are now shown in a better way. thanks!