Add a .tar.zst extension to ModelVersion downloads

https://redmine.teklia.com/issues/9362

A new arkindex.project.aws.S3FileMixin.s3_filename property should be defined, returning None by default. When it returns a string, the s3_url property should use it to set a ResponseContentDisposition to attachment; filename="{s3_filename}" when generating the presigned URL.

This new property can be overridden in the ModelVersion to set a .tar.zst extension, which should allow Windows 11 to extract them properly and XArchiver to open the TAR directly.

A unit test should check that the s3_url that DownloadModelVersion redirects to includes the expected response-content-diposition=… query parameter.