Evaluation: Add signed URL support

[vprashrex]

Is your feature request related to a problem?
Clients currently cannot download files directly as the GET /evaluations/datasets/{id} endpoint only returns dataset metadata without a download URL. This limits accessibility and usability for clients.

Describe the solution you'd like

• Add an include_signed_url=true query param.
• Generate a time-limited signed URL from AWS S3.
• Return the signed URL in a signed_url field.
• Skip signing if object_store_url is null.
• Return 404 if the dataset doesn't exist before attempting signing.

Original issue

Describe the current behavior
Current behavior: GET /evaluations/datasets/{id} returns dataset metadata but no download URL, clients can't directly download the file.

Describe the enhancement you'd like
Added include_signed_url=true query param that generates a time-limited signed URL from aws s3 and returns it in signed_url field.

Why is this enhancement needed?
Enables the client to view dataset file

Additional context
Skips signing if object_store_url is null; returns 404 before attempting signing if dataset doesn't exist.