Ensure that the path fed to MediaPipe contains only ASCII characters

[jorpelae]

Fix for https://github.com/homuler/MediaPipeUnityPlugin/issues/1284

MediaPipe seems to fail to load assets from folders containing Japanese characters on Windows. After debugging the C++ side, couldn't find any obvious problem without going into MediaPipe itself.

As a workaround, I added a step to check if the asset path contains non-ASCII characters, and if this is the case, make a copy to a temporary path.