Draft: Allow workers to query the total size when creating new files

This allows kio-http and kio-onedrive to avoid a memory copy.

Marked as draft because I am not sure if this is the best design, and it appears that the new metadata is only visible after the worker calls readData despite it being sent a little bit after canResume. Perhaps there is a way for the worker to freeze right after canResume and stay frozen until it gets the new metadata?