Refactor: ChatAgent

[WHALEEYE]

Description

Thoroughly refactored ChatAgent.

Motivation and Context

• Move function calling into model backends - OpenAI Supported
• Move structured output into model backends - OpenAI Supported
• Code quality improvement inside ChatAgent
• Support true asynchronous model calling
• Support tool calling for stream output

This will close #1334.

Important Changes

For the convenience of the reviewers, the important changes to the code are listed here:

1. rename functions: async functions are prefixed with a instead of being postfixed with async, for example, step_async -> astep, to make it more concise.
2. moved some of the internal function and types into _utils.py and _types.py.
3. added async version of stream handling.
4. added an internal type ModelResponse to represent the responsefrom models, which will only be used in internal ChatAgent.
5. removed all keys with None value in configs after as_dict().

[MuggleJinx]

Thanks Zhaoxuan! it's a lot of wrok! I left some small comments. It looks good to me, we can merge it soon if all test has been passed.