Make helpers that read from disk async

examples/test-device/src/main.rs

@@ -6,6 +6,7 @@ use tokio;
 #[tokio::main]
 async fn main() {
     let app_secret = yup_oauth2::read_application_secret(path::Path::new("clientsecret.json"))
+        .await
         .expect("clientsecret");
     let auth = DeviceFlowAuthenticator::builder(app_secret)
         .persist_tokens_to_disk("tokenstorage.json")