Async version of multiple sequential chat

[randombet]

Why are these changes needed?

This PR add a new a_initiate_chats interface to add concurrent execution in comparison to initiate_chats interface.
It allows one to pass multiple tasks and their corresponding dedicated agents and execute concurrently. Depending on the prerequisite task(s), the tasks will be solved in a concurrent manner.

Related issue number

Closes #1596

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.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 87.23404% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.13%. Comparing base (a34e4cc) to head (3864851).
⚠️ Report is 2490 commits behind head on main.

Files with missing lines	Patch %	Lines
autogen/agentchat/chat.py	87.20%	5 Missing and 6 partials ⚠️
autogen/agentchat/conversable_agent.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1724       +/-   ##
===========================================
+ Coverage   39.35%   59.13%   +19.77%     
===========================================
  Files          57       57               
  Lines        6093     6171       +78     
  Branches     1362     1502      +140     
===========================================
+ Hits         2398     3649     +1251     
+ Misses       3499     2235     -1264     
- Partials      196      287       +91     

Flag	Coverage Δ
unittests	59.09% <87.23%> (+19.74%)	⬆️

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[qingyun-wu]

The changes in the PR look great! Thank you @randombet! I think it would be even better if we also add methods such as add_chat, remove_chat which allow one to add or remove chat in the chat_queue concurrent to a_initiate_chats. This could be done in a separate PR!

[randombet]

The changes in the PR look great! Thank you @randombet! I think it would be even better if we also add methods such as add_chat, remove_chat which allow one to add or remove chat in the chat_queue concurrent to a_initiate_chats. This could be done in a separate PR!

Cool. Let's do it in another PR