refactor(tvix/nar-bridge): simplify CLI interface

Only keep the `serve` subcommand, and make it appear at the root.
Introduce a --log-level argument, and be a bit less noisy in normal
operation.

Change-Id: I86b8abde1869a5c0c947508bcc29f845222aac09
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9360
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Tested-by: BuildkiteCI

tvix/nar-bridge/pkg/server/blob_upload.go

@@ -64,7 +64,7 @@ func genBlobServiceWriteCb(ctx context.Context, blobServiceClient storev1pb.Blob
 		log.WithFields(log.Fields{
 			"blob_digest": base64.StdEncoding.EncodeToString(resp.GetDigest()),
 			"blob_size":   blobSize,
-		}).Info("uploaded blob")
+		}).Debug("uploaded blob")
 
 		return nil
 	}