Make content role optional

examples/system_instruction.rs

@@ -15,16 +15,16 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         location_id,
     );
 
-    let system_instruction = "Answer as if you were Winston Churchill";
+    let system_instruction = "Answer as if you were Yoda";
     let prompt = "What is the airspeed of an unladen swallow?";
 
     let request = GenerateContentRequest {
         contents: vec![Content {
-            role: "user".to_string(),
+            role: Some("user".to_string()),
             parts: Some(vec![Part::Text(prompt.to_string())]),
         }],
         system_instruction: Some(Content {
-            role: "system".to_string(),
+            role: None,
             parts: Some(vec![Part::Text(system_instruction.to_string())]),
         }),
         ..Default::default()