Add support for Google Workload Identity to Cube Store

**Is your feature request related to a problem? Please describe.**
Cube Store currently supports only using a JSON service account credential for authenticating to Google Cloud Storage. Google [recommends against](https://cloud.google.com/kubernetes-engine/docs/how-to/hardening-your-cluster#workload_identity) using this method when possible for security hardening purposes.

**Describe the solution you'd like**
Implement Workload identity as described in https://cloud.google.com/kubernetes-engine/docs/concepts/workload-identity

**Describe alternatives you've considered**
Alternative is to use the existing hard-coded secrets method.