Adding max_tokens to autogenstudio LLMConfig

[amihalik]

Why are these changes needed?

I am creating a demo using autogenstudio and writing out a dbdefaults.json. I need to set the max_tokens value and this is missing from the LLMConfig datamodel.

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.

[victordibia]

Thanks for the contribution.
In the future, it would be great to open a PR on the autogenstudio branch, following the autogenstudio contribution guide

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2a2e466) 35.07% compared to head (d04df7e) 35.07%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1574   +/-   ##
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  Coverage   35.07%   35.07%           
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  Files          44       44           
  Lines        5323     5323           
  Branches     1234     1234           
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  Hits         1867     1867           
  Misses       3301     3301           
  Partials      155      155           

Flag	Coverage Δ
unittests	35.07% <ø> (ø)

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