gltfio: add extended tangents job

[poweifeng]

• TangentsJobExtended extracts data from cgltf accessor and runs geometry::TangentSpaceMesh on the attributes and computes the tangent space.
• The /extended folder is meant for running this process. Note that this API might remesh the input and will require corresponding changes that might break previous assumptions.
• The general flow of the code is modeled after src/TangentsJob.h
• This is not hooked into current code and should have no practical effect on gltfio.