Adjusts send_introductions Type Hint in GroupChat

[WaelKarkoub]

Why are these changes needed?

Closes #1689

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

All modified and coverable lines are covered by tests ✅

Project coverage is 47.62%. Comparing base (78a5a6b) to head (87ca0a5).

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1912       +/-   ##
===========================================
+ Coverage   37.09%   47.62%   +10.53%     
===========================================
  Files          64       64               
  Lines        6834     6834               
  Branches     1502     1629      +127     
===========================================
+ Hits         2535     3255      +720     
+ Misses       4082     3311      -771     
- Partials      217      268       +51     

Flag	Coverage Δ
unittests	47.49% <100.00%> (+10.40%)	⬆️

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[ekzhu]

Is there a reason to make it a Draft?

[WaelKarkoub]

@ekzhu just wanted to make sure #1906 gets merged in first