Work around Go client OneOf unmarshaling bug for file_metadata

The Go client's OneOf unmarshaler for file_metadata fails because both CloudStorage and LocalFile types can match the JSON response (LocalFile has AdditionalProperties that accepts any extra fields). This causes the unmarshaler to set UnparsedObject instead of populating the typed fields.

Workaround:
- Manually parse UnparsedObject when it's set, checking for 'access_details' to distinguish CloudStorage from LocalFile
- Skip utils.CheckForUnparsed() for reference tables since file_metadata will always have UnparsedObject set
- Update data source to handle Values as interface{} (API spec change)
- Remove LocalFile.upload_id field (no longer in API)

Author: g-brizolier