process message before send

[sonichi]

Why are these changes needed?

• Add a hookable method for processing a message before sending. Example application: customized frontend to display messages .
• Renamed other hookable methods for clarity.

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

Attention: Patch coverage is 89.47368% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 48.16%. Comparing base (085bf6c) to head (9f2fea1).

Files	Patch %	Lines
...agentchat/contrib/capabilities/context_handling.py	0.00%	1 Missing ⚠️
...gen/agentchat/contrib/capabilities/teachability.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1783       +/-   ##
===========================================
+ Coverage   36.92%   48.16%   +11.23%     
===========================================
  Files          62       62               
  Lines        6492     6499        +7     
  Branches     1437     1561      +124     
===========================================
+ Hits         2397     3130      +733     
+ Misses       3902     3120      -782     
- Partials      193      249       +56     

Flag	Coverage Δ
unittests	48.08% <89.47%> (+11.16%)	⬆️

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[gagb]

This is really really useful!