artifact -> attestaton -> provenance linkage #415
Description
This task is about being able to ask about any information we know about a given software artifact by its content digest.
For example, you could run /discover/sha256-deadbeef and you'll get the attestations associated with a container image with digest (sha256-deadbeef) and also its related pieces of evidence (attestations, SBOMs, ...)
Conceptually this is similar to what the referrer API does in OCI but across all cas-backends and Chainloop organizations.
NOTE: in a future PR, we might want to propagate referrer information to OCI (if that's the backend that's being used) so users can leverage OCI clients such as regclient, oras, and so on.
### Tasks
- [x] database denormalization https://github.com/chainloop-dev/chainloop/pull/416
- [x] discovery endpoint https://github.com/chainloop-dev/chainloop/pull/424
- [x] CLI support https://github.com/chainloop-dev/chainloop/pull/424