Replace print commands with structured messages

[kumaranvpl]

Why are these changes needed?

Related issue number

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• I've included any doc changes needed for https://docs.ag2.ai/. See https://docs.ag2.ai/docs/contributor-guide/documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[sonichi]

Looks good. Could you check whether any doc change is needed, for example, about the interface to get usage summary? cc @yiranwu0 @marklysze

[sonichi]

Suggestion: add docstr to explain how to use this class in a number of steps for developers who need to define a new message type. For example, 1. define a subclass with a few data fields and override the print method, 2. define a function to create an instance of the subclass, like create_..., 3. In agentchat, when ..., do ....

Question: Can this class support multimodal messages, such as image, audio, video? cc @BabyCNM

[davorrunje]

This is just the first part of the task, the next one is to refactor IOStream classes to use structured messages, especially with WebSockets. We'll write a blog post on how to use it to build UI easier.

[davorrunje]

We'll have to add multimodal messages in another PR. It needs to be supported by client classes in a unified way first.

[davorrunje]

I create the followup issue #331

[kumaranvpl]

Looks good. Could you check whether any doc change is needed, for example, about the interface to get usage summary? cc @yiranwu0 @marklysze

No docs change needed because I didn't modify signature of existing functions and methods. I just moved the print statements to single place.

I am not done yet though, still there are few places to change.