Add logging of blobs being processed to implementations of the BlobAttributeBackfillService

When the replication.blobattributesbackfill task runs, it invokes BlobAttributeBackfillService.void backfillBlobAttributes(BlobStore blobStore, BlobId blobId);

There is no logging in the task or this service which reports what blob is about to be processed. This makes it impossible to know what file could have a problem, if during processing there is a failure.

Expected

Add logging of blobs about to be processed at TRACE, and log at TRACE this information only to the replication.blobattributesbackfill task log, not the nexus.log ( application log)