Added support for Option types

[ivanleomk]

We can now support option types with this new PR

let client = Client::new(env::var("OPENAI_API_KEY").unwrap().to_string());
        let instructor_client = from_openai(client);

        #[derive(InstructMacro, Debug, Serialize, Deserialize)]
        struct UserInfo {
            name: String,
            age: u8,
        }

        #[derive(InstructMacro, Debug, Serialize, Deserialize)]
        struct MaybeUser {
            #[description("This is an optional user field. If the user is not present, the field will be null")]
            user: Option<UserInfo>,
        }

        let req = ChatCompletionRequest::new(
            GPT4_O.to_string(),
            vec![chat_completion::ChatCompletionMessage {
                role: chat_completion::MessageRole::user,
                content: chat_completion::Content::Text(String::from("It's a beautiful day out")),
                name: None,
            }],
        );

        let result = instructor_client
            .chat_completion::<MaybeUser>(req, 3)
            .unwrap();

        assert!(result.user.is_none());

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