Support generic kwargs to cloud storage client SDKs

[Mchristos]

The idea behind this issue is to generically support keyword arguments to client library functions.

For example, in google-cloud-storage, many functions support common kwargs such as timeouts and retry confugurations. See this PR for a non-generic implemetation of these two kwargs.

This change would be to provide a generic mechanism for supporting kwargs from different cloud providers, in various functions involving, for example, downloading and uploading files.