Add an example using autogen-core and FastAPI to create streaming responses

[ToryPan]

Why are these changes needed?

This PR adds an example demonstrates how to build a streaming chat API with multi-turn conversation history using autogen-core and FastAPI.

Related issue number

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• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md 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.

[ToryPan]

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[SongChiYoung]

@ToryPan
Cool work!
If you get a chance, could you take a look at my third-party extension project autogen-oaiapi?

I think some of your ideas from this example could really help improve it.
Even just sharing issues or thoughts would be super helpful.

Would love to get your thoughts!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.97%. Comparing base (9b0a0bd) to head (952b2bb).
Report is 1 commits behind head on main.

Additional details and impacted files

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##             main    #6335   +/-   ##
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  Coverage   77.97%   77.97%           
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  Files         214      214           
  Lines       15377    15377           
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  Hits        11990    11990           
  Misses       3387     3387           

Flag	Coverage Δ
unittests	77.97% <ø> (ø)

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