feat(gc): improve logging for artifact deletions

This MR improves logging for when an artifact (blob or manifest) is deleted by online GC.

For manifests, so far we only logged the DB manifest ID for the deleted manifest. This would then require a separate lookup to find the manifest digest. This MR adds a new manifest deleted message at the end which includes a key/value pair for the digest. To make this possible we change the manifest delete query which is exclusively used by the online garbage collector so that it returns the digest of the deleted manifest (if found), avoiding the need for a separate SELECT to obtain it.

For consistency, we added a new blob deleted message at the end of the deletion for blobs as well. It is already possible to find the digest of a deleted blob in the logs, but this requires using the correlation ID for the GC run, as the digest is only logged once. This will make it easier to grep logs in bulk.