Python: feat: implement ParallelToolExecutor for concurrent tool calling #4429

[jhawpetoss6-collab]

This PR implements the ParallelToolExecutor for the Microsoft Agent Framework, addressing the need for parallel tool calling in declarative workflows (#4429).

Changes:

• Added ParallelToolExecutor with configurable concurrency limits.
• Uses asyncio.gather and asyncio.to_thread for non-blocking execution.
• Essential for complex agents performing multiple tool-based lookups or actions simultaneously.

/claim #4429

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[eavanvalkenburg]

I'll let @moonbox3 and @TaoChenOSU comment on if we need this, because I kind of doubt it, we already support fan-in/fan-out, but this is the definitely not the place to put this.