Add glTF2 generator to core libraries

[johnwason]

glTF2 is a portable 3D model format that is supported by most modern renderers including Three.js, QT, Unity, etc. The format is fairly simple, and I implemented a utility in Python that converted a Tesseract scene to glTF, and then loaded the scene into a browser for visualization. I want to have this functionality available in tesseract_robotraconteur, and I think it makes sense to add it to the core libraries since it is relatively simple.

Here is the Python implementation:

https://github.com/tesseract-robotics/tesseract_python/blob/master/tesseract_viewer_python/tesseract_robotics_viewer/tesseract_env_to_gltf.py