Add support for default model client, in AGS updates to settings UI

[victordibia]

Why are these changes needed?

Add support for default model client, in AGS updates to settings UI.
A default model client will be used for subsequent background tasks (e.g, automatically naming sessions to meaningful names).

Related issue number

Closes #5685

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com./microsoft/autogen/blob/main/CONTRIBUTING.md 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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.96%. Comparing base (07a455f) to head (6447ffa).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5763   +/-   ##
=======================================
  Coverage   74.96%   74.96%           
=======================================
  Files         175      175           
  Lines       11158    11158           
=======================================
  Hits         8365     8365           
  Misses       2793     2793           

Flag	Coverage Δ
unittests	74.96% <ø> (ø)

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