Support backends/clients being used as context managers

Now that we have the `CloudPath` factory on the client, it can be nice to have a scoped backend that you don't have to worry about conflicting with other backends.

This may be as easy as defining `__enter__` and `__exit__`, but we should think if there are any resources that would be scoped to the context. Might be more relevant in backends/clients with persistent connections, etc.