Implement 'candidate_func' parameter to filter down the pool of candidates for selection

[Ethan0456]

Summary of Changes

• Added 'candidate_func' to 'SelectorGroupChat' to narrow-down the pool of candidate speakers.
• Introduced a test in tests/test_group_chat_endpoint.py to validate its functionality.
• Updated the selector group chat user guide with an example demonstrating 'candidate_func'.

Why are these changes needed?

• These changes adds a new parameter candidate_func to SelectorGroupChat that helps user narrow-down the set of agents for speaker selection, allowing users to automatically select next speaker from a smaller pool of agents.

Related issue number

Closes #5828

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.

[Ethan0456]

@microsoft-github-policy-service agree

[ekzhu]

Please follow the developer guide here: https://github.com/microsoft/autogen/blob/main/python/README.md for fixing formatting and type checking errors.

cd python/packages/autogen-agentchat
poe pyright
poe mypy
poe format
poe lint

# Do the same for python/packages/autogen-core, where the docs are.

[Ethan0456]

Hi, I missed to lint the docs, sorry!

[ekzhu]

I fixed a bug in the select speaker flow.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 76.67%. Comparing base (8f8ee04) to head (89c0127).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...gentchat/teams/_group_chat/_selector_group_chat.py	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5954      +/-   ##
==========================================
- Coverage   76.67%   76.67%   -0.01%     
==========================================
  Files         191      191              
  Lines       13141    13149       +8     
==========================================
+ Hits        10076    10082       +6     
- Misses       3065     3067       +2     

Flag	Coverage Δ
unittests	76.67% <83.33%> (-0.01%)	⬇️

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[Ethan0456]

Thanks @ekzhu, for all your help with my first PR! I really appreciate your guidance.