Remove gogoproto.customtype

gogoproto.customtype is used to have go-digest.Digest instead of string.
While it is convinient, protoc-gen-go doesn't support the extension
and that blocks #6564.

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

content/proxy/content_reader.go

@@ -36,7 +36,7 @@ func (ra *remoteReaderAt) Size() int64 {
 
 func (ra *remoteReaderAt) ReadAt(p []byte, off int64) (n int, err error) {
 	rr := &contentapi.ReadContentRequest{
-		Digest: ra.digest,
+		Digest: ra.digest.String(),
 		Offset: off,
 		Size_:  int64(len(p)),
 	}