feat(storage): add support for generators for upload body

[langma]

Add explicit support for generators, async and sync, to support other than
existing IOBase or file upload sources.

Fixes #206

[langma]

I tried it locally and can say that the async version is behaving like I thought, allowing me to remove the boilerplate buffering code. However, I haven't tried with the REST version (although requests should also accept a Generator body). Also, I'm using python 3.8 and cannot easily test older versions.