Add file uploader for SLSA attestations

This issue is focused on adding file uploader capabilities to the new SLSA attestations model described in Create database table for SLSA attestations (#554590 - closed) and Add DB model for Attestations (#554591 - closed). See https://gitlab.com/gitlab-org/gitlab/-/issues/553213+ for more details on the proposed workflow and design.

Since attestation bundles can be larger than 64k, we should add the ability to store those files using the standard file storage mechanisms supported by GitLab (object storage / local storage).

A file upload limit size of 100M should be sufficient.