Create Attestations Download Route

In order for SLSA attestations to be downloadable via the UI, a download route will need to be added. The scope of this issue is build out the implementation described below.

The route should be available at the path /:project_path/attestations/:iid/download and should download the attestation file to the users machine.

The visibility should match that of the project, so if the project is public, the attestation should be publicly visible.

This change will also require that iid support is added to the attestations table in the database.