Support async nested chats

[heyitsaamir]

In this PR:

1. We add a new use_async flag to register_nested_chats function. This allows the nested chats to internally run a_initiate_chats instead of initiate_chats.
2. Added a number of non-open-ai dependent tests for nested chats.

Why are these changes needed?

We support nested chats and async initiate_chats. However, if a chat is started asynchronously, nested chats still currently run in-process.

Related issue number

Closes #3286

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation 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.

[qingyun-wu]

@heyitsaamir, thanks for the contribution! I know this is still a draft PR, but several reminders:

• Could you first fix the code format issues?
• Tests should be added to cover the added code.

[heyitsaamir]

@heyitsaamir, thanks for the contribution! I know this is still a draft PR, but several reminders:

• Could you first fix the code format issues?
• Tests should be added to cover the added code.

Yep. Working on tests. I am surprised there are no existing tests for nested chats.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 5.00000% with 38 lines in your changes missing coverage. Please review.

Project coverage is 18.48%. Comparing base (6279247) to head (cd454e4).
Report is 69 commits behind head on main.

Files	Patch %	Lines
autogen/agentchat/conversable_agent.py	5.00%	38 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3289       +/-   ##
===========================================
- Coverage   32.90%   18.48%   -14.42%     
===========================================
  Files          94      103        +9     
  Lines       10235    10932      +697     
  Branches     2193     2498      +305     
===========================================
- Hits         3368     2021     -1347     
- Misses       6580     8709     +2129     
+ Partials      287      202       -85     

Flag	Coverage Δ
unittests	18.45% <5.00%> (-14.46%)	⬇️

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[heyitsaamir]

Closing in favor of #3309