Add support for cancellation

Using LLMs can be expensive or slow. Add support for cancellation - https://docs.rs/tokio-util/latest/tokio_util/sync/struct.CancellationToken.html

Seems like in JS, they use the 2nd args for options - https://js.langchain.com/docs/modules/model_io/chat/cancelling_requests

could also support none as option.

```rust
let open_ai = OpenAI::default();
let response = open_ai.invoke("What is the capital of France?", None).await.unwrap();
```

```rust
let token = CancellationToken::new();

let open_ai = OpenAI::default();
let response = open_ai.invoke("What is the capital of France?", InvokeOptions::default().with_cancellation(token.clone())).await.unwrap();
```