[Issue]: Simpler specification of nested chat

[sonichi]

Describe the issue

Nested chat is a common way to create complex chats, meaning that an agent will reply after another conversation. It can be used in hierarchical chat, joining multiple chats etc. Examples: https://github.com/microsoft/autogen/blob/main/notebook/agentchat_chess.ipynb https://github.com/microsoft/optiguide https://github.com/microsoft/autogen/blob/main/notebook/agentchat_planning.ipynb https://github.com/microsoft/autogen/blob/main/notebook/agentchat_two_users.ipynb

It may be valuable to offer syntax convenience to simplify the spec of such nested chat. As a validation, the above examples should end up with a simple and intuitive implementation.

Steps to reproduce

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Additional Information

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