Add support for the Google Search tool

- Gemini 2.0 and above use a google_search tool instead of a
google_search_retrieval tool. This commit adds support for the new tool.

examples/google-search-retrieval.rs

@@ -29,8 +29,13 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         ..Default::default()
     };
 
+    println!(
+        "Request: {}",
+        serde_json::to_string_pretty(&request).unwrap()
+    );
+
     let result = gemini
-        .generate_content(&request, "gemini-1.0-pro-002")
+        .generate_content(&request, "gemini-1.5-flash-002")
         .await?;
 
     println!("Response: {:?}", result.candidates[0].get_text().unwrap());