Add example for parallel agents

[ZHANG-EH]

Why are these changes needed?

This PR adds an example of parallel-agents that runs multiple instances of Magentic-One in parallel, with support for early termination and final answer aggregation.

Related issue number

Checks

• 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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.90%. Comparing base (e49ee48) to head (dd7c2f6).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6311   +/-   ##
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  Coverage   77.90%   77.90%           
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  Files         209      209           
  Lines       15189    15189           
=======================================
  Hits        11833    11833           
  Misses       3356     3356           

Flag	Coverage Δ
unittests	77.90% <ø> (ø)

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