Store attestations in CAS

[danlishka]

I would like to have the option to download attestations in the same way we currently can get materials.

chainloop artifact download --digest sha256:9be6e32ee1715a759...

With just one sha256, this will allow me to share easily:

• attestations,
• all referenced materials and artifacts.

We will need to start storing attestations in CAS as we do in case of integrations (please check the attached screenshot).

[migmartri]

Makes sense. Note that we used to store attestations in the CAS backend before #180 but moved to DB.

The main difference between before and now, is that we want to add the attestation just for download purposes and in addition to DB, which will not make it a requirement per-se I think.