[Feature Request] ChatAgent further refactor

[Wendong-Fan]

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Motivation

further enhancement based on #1142

1. Update dynamic tool calling mechanism.
2. Implement exception handling for memory overflow (refer to: GitHub PR discussion).
3. Relocate tool calling logic from the agent level to the model level.
4. Add check for the necessity of tool calling to help exiting while loop in .step.
5. decouple the structure output from the agent somehow, like the tool calling mechanism.
6. Asynchronous support (Model and Agent level).
7. Enable streaming output.

Solution

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Alternatives

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Additional context

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