[Feature Request] Refactor ChatAgent

[Wendong-Fan]

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Motivation

1. current ChatAgent is redundant, we want to refactor this class to make it more tidy
2. we need to support passing API Key as parameter into ChatAgent , this feature is required for the platform development
3. remove ChatGPTVisionConfig , merge FunctionCallingConfig into ChatGPTConfig to make the abstraction tidy as discussed in feat: enable image modality for ChatAgent #473

Solution

1. Separate initialization of model and chat agent
2. model_type ,model_config ,message_window_size,token_limit move to model
3. move def handle_batch_response and def handle_stream_response from chat agent to model
4. add config folder, set model config based on framework done by Zecheng
5. Model.create(platform='', model='',...)

Alternatives

No response

Additional context

No response

[Wendong-Fan]

@ocss884 @zechengz feel free to add content into this issue description