Feature request: garbage collection for orphan materials

[jiparis]

During the attestation process, materials are uploaded to the storage backend through the CAS service. In cases where the attestation fails or is cancelled, those materials are kept forever in the storage backend.

It would be useful (from the storage point of view) to have a process that checks for those unfinished attestations and removes the materials if they are not referenced by any other attestation.

[javirln]

Hmm is it something that https://cloudcustodian.io could potentially do? 🤔