Add assistant api

examples/chat_completion.rs

@@ -1,12 +1,13 @@
 use openai_api_rs::v1::api::Client;
 use openai_api_rs::v1::chat_completion::{self, ChatCompletionRequest};
+use openai_api_rs::v1::common::GPT4;
 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 = ChatCompletionRequest::new(
-        chat_completion::GPT4.to_string(),
+        GPT4.to_string(),
         vec![chat_completion::ChatCompletionMessage {
             role: chat_completion::MessageRole::user,
             content: String::from("What is Bitcoin?"),