refactor(tvix/nar-bridge): let callbaks return calculated digests

This aligns behaviour more with how it should be - it's the
responsibility of the callback functions to return digests of the things
they consume(d). It allows further cleaning up the hasher struct.

Change-Id: I9cbfc87e6abd4ff17fadf39eb6563ec3cb7fcc6f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9528
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: Connor Brewster <cbrewster@hey.com>

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

@@ -44,7 +44,7 @@ func registerNarPut(s *Server) {
 			// buffer the body by 10MiB
 			bufio.NewReaderSize(r.Body, 10*1024*1024),
 			genBlobServiceWriteCb(ctx, s.blobServiceClient),
-			func(directory *castorev1pb.Directory) error {
+			func(directory *castorev1pb.Directory) ([]byte, error) {
 				return directoriesUploader.Put(directory)
 			},
 		)