Implement partial or streaming reads/writes (CloudFile abstraction)

Currently, if you want to read a file we download the whole file and then open it for reading. Most backends will have some way to do streaming reads. We may be able to improve the experience if we can do the same.

There may be some tricky bits with caching here. Can we stream to the user and then cache the streamed portion at the same time? Is there an async way that makes sense to do this?