Add stream method to object store

[timteeee]

Branched off of https://github.com/roeap/object-store-python/pull/28

Here I am adding a new stream method to the ObjectStore class that returns a chunked stream over the bytes as an async iterator. Useful for large objects that cannot reasonably fit into memory all at once.

[kylebarron]

I hope you don't mind I'm incorporating this PR into my alternative binding: https://github.com/developmentseed/object-store-rs/pull/11

[timteeee]

@kylebarron absolutely don't mind, please do!