Support guest/unauthorized user?

Is there a way to allow un-authorized users for read-only operations?

Similar to -> https://github.com/aws-amplify/amplify-js/issues/711

```
const client = new AWSAppSyncClient({
    url: AppSync.graphqlEndpoint,
    region: AppSync.region,
    auth: {
        // Amazon Cognito Federated Identities using AWS Amplify
        credentials: () => Auth.currentCredentials()
    },
});
```