Re-commit of #2182: Customisation of speaker select message and prompt in GroupChat

[marklysze]

Changes committed as per #2182 to a branch within autogen repository.

Please see #2182 for the reason and code changes / revisions for "Customisation of speaker select message and prompt in GroupChat".

Related issue number

This PR replaces #2182.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 57.06%. Comparing base (54b5bbb) to head (5027790).

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2233       +/-   ##
===========================================
+ Coverage   37.85%   57.06%   +19.21%     
===========================================
  Files          77       77               
  Lines        7769     7780       +11     
  Branches     1664     1803      +139     
===========================================
+ Hits         2941     4440     +1499     
+ Misses       4579     2974     -1605     
- Partials      249      366      +117     

Flag	Coverage Δ
unittest	14.37% <15.38%> (?)
unittests	56.04% <100.00%> (+18.19%)	⬆️

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