Feature request: support queue_derive in modify_metadata

[p8NG6L]

Here's my use case: I have an item to upload with multiple files and metadata that varies per file. So I'm calling upload(id, files=files, metadata=metadata) once with the item metadata, then repeatedly calling modify_metadata(id, target="files/<filename>", metadata=<file_metadata>) for each of the files. I'd like to not start a derive until the entire process is finished. I can set queue_derive=False on the upload call, but modify_metadata doesn't take a queue_derive or headers argument, so there's no obvious way to start the derive task short of re-uploading one of the files (e.g., the smallest one).

I would also be happy if someone can suggest some no-op operation that would allow me to queue a derive as a side effect.

[p8NG6L]

Item.derive in v1.9.3 and the ia tasks <identifier> --cmd derive.php command mentioned at #351 (comment) solve this issue for me.