[.Net] [Feature Request]: Add ReAct sample

[LittleLittleCloud]

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[qingyun-wu]

@LittleLittleCloud, for your reference: this is an ReAct example with pyautogen: https://microsoft.github.io/autogen/docs/topics/prompting-and-reasoning/react

[LittleLittleCloud]

@qingyun-wu Thanks, there are slight difference on the purpose of the two examples thought. This example is to show how to implement ReAct as a single agent and the ReAct example in pyautogen uses two-agent chat conversation pattern.