Add support for KHR_materials_unlit and add necessary extensions serialization plumbing

[RunitaiLinden]

Test plan

1. Download each of the assets from the glTF folder of the Test model.
2. Create a box in Second Life.
3. While the box is selected in edit mode, click Develop menu > GLTF > Open....
4. Choose the .gltf file of the Test model.
5. Verify the model appears in-world and matches the Screenshot on Test model page.

Description

See KHR_materials_unlit

Test model:
https://github.com/KhronosGroup/glTF-Sample-Models/tree/main/2.0/UnlitTest

[katerynamproductengine]

Failed QA.

On OSX - Passed QA. The model appears in-world and matches the Screenshot on Test model page.
On Win10 - Failed QA. Incorrect white display of the entire viewer menu after choosing the .gltf or .glb file of the Test model. See this issue #1894 .

Verified on the Second Life Release 7.1.9.9620320242 (64bit) on Win10/OSX in the scope of https://github.com/secondlife/iqa/issues/266.