feat(controlplane): dynamic CAS backend selection during download

[migmartri]

Enables download endpoints to find the CAS backend that contains a given digest. This solves the limitation we had of downloading from currentOrgs and default backends only. Now an user will be able to download an item from any cas-backend in any of their orgs.

The selection process works as follows:

• Find the casBackends where we have a record that the digest was uploaded, this is across all the organizations
• Then match these results with the organizations our user has access to (memberships)
• if we still have a match, means that we can download the item from one or more cas-backends
• If we have a single cas-backend or a default one we return that
• Otherwise the first one

This download selection works for both the new /download/sha256:[] endpoint and for the artifact download CLI command.

Closes #268

[danlishka]

Thank you.