Should allow user to configure the client with better flexibility

[jason-zhj]

Is your feature request related to a problem? Please describe. Some organisations may have their own GPT proxy, so when using this library, they would need to specify their proxy url. In this case, seems NewClientWithConfig is the only function that they can use, but right now the authToken field in ClientConfig is not exposed, so they won't be able to pass in auth token. So right now NewClientWithConfig is basically unusable.

Describe the solution you'd like Change authToken in ClientConfig into an expose field. Then NewClientWithConfig becomes usable