Allow usage w/ an emulator

[crepererum]

It would be nice if cloud-storage would allow users to connect to an emulator like https://github.com/oittaa/gcp-storage-emulator or https://github.com/fsouza/fake-gcs-server for local testing. For this the following behavior changes are required:

1. Disable auth. Other libs call this AnonymousCredentials, but I think it's just that they don't specify any auth headers at all (ref).
2. Allow to specify a custom endpoint instead of a hardcoded BASE_URL.

[ThouCheese]

Sure this is something that we can do. Do you have a specific API in mind that works well for you? Maybe something like

cloud_storage::Client::new()
    .disable_auth()
    .base_url("whatever")
    // rest of your behaviour

Would that work/be convenient?

[crepererum]

The proposed API looks perfect. 👍