Change ArnoldUsd filename attribute to be asset instead of filename

[sebastienblor]

Currently the usd procedural attribute filename is considered as a string attribute (it doesn't have the arnold metadata "path" set to "file").
This metadata is needed for the usd writer to author it as an asset.
The schema is therefore declaring it as a string too.
Ideally this should be an asset, as for other file attributes

[autodesk-oss-arnold-bot]

Issue synced internally to ARNOLD-15658