feat: add chain method to create Request instance

examples/embedding.rs

@@ -4,11 +4,12 @@ use std::env;
 
 fn main() -> Result<(), Box<dyn std::error::Error>> {
     let client = Client::new(env::var("OPENAI_API_KEY").unwrap().to_string());
-    let req = EmbeddingRequest {
-        model: "text-embedding-ada-002".to_string(),
-        input: "story time".to_string(),
-        user: Option::None,
-    };
+
+    let req = EmbeddingRequest::new(
+        "text-embedding-ada-002".to_string(),
+        "story time".to_string(),
+    );
+
     let result = client.embedding(req)?;
     println!("{:?}", result.data);