[Unity plugin] Add support for OBJ file import from MuJoCo scenes

[Balint-H]

This feature handles OBJ mesh assets referenced in MJCF. It applies the scale to the OBJ mesh then saves it as a local asset in the project, after swizzling the mesh coordinates. An additional step is taken to enable read-write of mesh data from obj files. We flip around mesh faces if the scale has an odd number of negative numbers. In fact this is a missing detail from the STL parser, will add a fix for that soon in a separate PR.

A related feature included is by default now the mesh renderer is turned of Geoms with group >=3 (default function with Simulate). This will make imported scenes appear more similar to what is expected from the Simulate UI (commonly collision geoms are hidden by default using this approach).

This now enables importing many more menagerie models to Unity.

Addresses #2372