usdTriFileFormat: Read .triangle file contents to determine payload expansion behavior

[rlei-weta]

Currently the usdTriFileFormat plugin registers a Usd_Triangle_SideLength metadatum which is consumed by the file format plugin to determine the side length of the triangular mesh. The file format plugin does not actually read any of the contents of the file payload itself.

A suggestion during a USD WG is to actually encode some meaningful information in the .triangle payload to determine some custom payload expansion behavior.

For example, one idea would be to encode 3 floats in the .triangle file, which can be read then authored as displayColor primvar on the mesh.