Support using a token with a custom api_root

[Kepnu4]

When I use gcloud.aio.storage.Storage with a custom api_root, it automatically sets the internal flag _api_is_dev = True, which causes the client to send an empty header to the endpoint.

Right now, I have to work around this by manually overriding the flag:

storage = Storage(api_root=url)
storage._api_is_dev = False

It would be great to have an official option to disable this behavior (e.g., a parameter in the constructor) instead of relying on a private attribute.