content parameter of TextMessage class can also be list

[rjambrecic]

Why are these changes needed?

Fixes Pydantic error breaking MultimodalConversableAgent

When using the MultimodalConversableAgent, the create_received_message_model function occasionally attempts to construct a TextMessage by setting its content to a structure like [{'type': 'text', 'text': '...'}].

As a result, to accommodate this structure in TextMessage.content, the type annotation should be updated to include list[dict[str, str]].

Related issue number

Closes #383

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