Notably, there were several code paths which could end up converting to
strings, sometimes multiple times, when passing data into USD APIs.
Reduce the use of raw `char *` in the asset interface and use
`std::string` instead to match what will be passed into USD. Using
string views, of any variety, will not help here since the APIs don't
natively support those.
Pull Request: https://projects.blender.org/blender/blender/pulls/140997
72de50483e