Enable the public access to materials and attestations in CAS

[danlishka]

Today all materials and attestations are available only to members of an organization in Chainloop. You need to first authenticate via chainloop auth login to use later chainloop artifact download ....

We want to allow setting the attestation and materials public at the attestation init stage:

chainloop attestation init --public

This way, for example, we will enable users to download and verify all attestations & materials produced during the Chainloop build and release process.

[danlishka]

What if we also add support for directly downloading artifacts through an URL? For example:

d.chainloop.dev/9be6e32ee1715a759...