GCS Support for Non-Public Buckets

[agchang]

Apache Iceberg version

0.3.0

Please describe the bug 🐞

I couldn't figure out how to get GCS as a file provider to work, when using buckets that are non-public. I see my credentials being configured, but I'm pretty part means we don't use it: https://github.com/apache/iceberg-go/blob/main/io/gcs.go#L58

[agchang]

Here is a PR that works with credentials, but let me know if I'm missing something about an existing use-case. #454

[loicalleyne]

@agchang GCS client uses Application Default Credentials; if you are running on a GCP VM this is automatically set, otherwise see here how to set.
https://gocloud.dev/howto/blob/#gcs

blob.OpenBucket will use Application Default Credentials; if you have authenticated via gcloud auth application-default login, it will use those credentials. See Application Default Credentials to learn about authentication alternatives, including using environment variables.

[rambleraptor]

I've got #579 to help implement AuthManager support. My goal is that AuthManager support will allow us to build authentication managers for many different providers (including Google), so that we can more natively support all of these different auth mechanisms.