Add sources field to TextMentionTermination

[Leon0402]

Why are these changes needed?

See #4807. In many practical scenarios, we only want to stop a dialog, when a specific agent says something e.g. not stop when the keyword is part of the prompt for instance. This PR allows this by adding a list of sources, where a user can specify the agent names.

Related issue number

Closes #4807

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]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 70.38%. Comparing base (34bc82e) to head (fe28aad).

Files with missing lines	Patch %	Lines
.../src/autogen_agentchat/conditions/_terminations.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5106      +/-   ##
==========================================
- Coverage   70.39%   70.38%   -0.02%     
==========================================
  Files         174      174              
  Lines       11014    11017       +3     
==========================================
+ Hits         7753     7754       +1     
- Misses       3261     3263       +2     

Flag	Coverage Δ
unittests	70.38% <75.00%> (-0.02%)	⬇️

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[ekzhu]

Would be good to update the unit test to harden the code