Feature request: option to auth to GCS with Application Default Credentials

Hi,

The Google Cloud Client Libraries allows for authenticating with services using Application Default Credentials. With this feature duplicacy running eg on a GCE instance could just connect to GCS bucket that stores repository using service account GCE instance runs as, without having to additionally fetch and maintain the file with credentials.

Adding this should be easy: extend duplicacy_gcsstorage.go:CreateGCSStorage with option to also call DefaultTokenSource.

Thanks!