Describe your feature request

In some cases, users may have access through application default credentials to query a BigQuery database but not have the ability to make an auth.json file. E.g., for the AllOfUs dataset research project, users can access the underlying data through BigQuery via application default credentials on the google compute engine servers they can access, but their accounts do not have sufficient permissions to make an auth.json file, to the best of my knowledge. It would be nice to have the interface enable constructing a bigquery client from application default credentials in those cases.