Allow limiting the maximum number of turns in initiate_chat and initiate_chats

[qingyun-wu]

Why are these changes needed?

This PR adds an argument in initiate_chat to allow users to limit the maximum number of conversation turns

Related issue number

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

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (bf3cc0f) 39.62% compared to head (9fbfacb) 58.42%.

Files	Patch %	Lines
autogen/agentchat/conversable_agent.py	84.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1703       +/-   ##
===========================================
+ Coverage   39.62%   58.42%   +18.79%     
===========================================
  Files          57       57               
  Lines        6006     6020       +14     
  Branches     1338     1464      +126     
===========================================
+ Hits         2380     3517     +1137     
+ Misses       3433     2222     -1211     
- Partials      193      281       +88     

Flag	Coverage Δ
unittests	58.40% <85.71%> (+18.77%)	⬆️

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