Change ArnoldUsd filename attribute to be asset instead of filename

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

Locate the ArnoldUsd procedural filename attribute and its schema declaration, then trace how the USD writer interprets the Arnold metadata. Confirm the attribute is authored as an asset rather than a string, matching the behavior expected for other file attributes.

Written by the indexing model from the issue text.