Add shortcuts for commonly used functions

examples/text-from-text.rs

@@ -16,12 +16,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
 
     let prompt = "What is the airspeed of an unladen swallow?";
     let request = GenerateContentRequest::builder()
-        .add_content(
-            Content::builder()
-                .role(Role::User)
-                .add_part(Part::Text(prompt.to_string()))
-                .build(),
-        )
+        .add_text_content(Role::User, prompt)
         .build();
     let response = gemini.generate_content(&request, "gemini-pro").await?;
     println!("Response: {:?}", response.candidates[0].get_text().unwrap());