Update GCP Auth to 0.12

examples/system_instruction.rs

@@ -1,13 +1,9 @@
-use std::sync::Arc;
-
 use gemini_rs::prelude::*;
 
-use gcp_auth::AuthenticationManager;
-
 #[tokio::main]
 async fn main() -> Result<(), Box<dyn std::error::Error>> {
     tracing_subscriber::fmt().init();
-    let authentication_manager = Arc::new(AuthenticationManager::new().await?);
+    let authentication_manager = gcp_auth::provider().await?;
     let api_endpoint = std::env::var("API_ENDPOINT")?;
     let project_id = std::env::var("PROJECT_ID")?;
     let location_id = std::env::var("LOCATION_ID")?;