Enable gemini context caching

[yeounoh]

Why are these changes needed?

Gemini model API introduced a new context caching feature that caches the prompt prefix. This PR implements enabled this new feature in GeminiClient to help reduce the cost of using the latest gemini models. Note that this is a gemini specific feature and used for caching the prompt prefix, not agent's input and output.

Related issue number

Addresses/closes #3038

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.

[yeounoh]

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 43.47826% with 26 lines in your changes missing coverage. Please review.

Project coverage is 13.82%. Comparing base (6279247) to head (37cd8b5).
Report is 47 commits behind head on main.

Files	Patch %	Lines
autogen/oai/gemini.py	43.47%	26 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3207       +/-   ##
===========================================
- Coverage   32.90%   13.82%   -19.09%     
===========================================
  Files          94       97        +3     
  Lines       10235    10849      +614     
  Branches     2193     2488      +295     
===========================================
- Hits         3368     1500     -1868     
- Misses       6580     9313     +2733     
+ Partials      287       36      -251     

Flag	Coverage Δ
unittests	13.78% <43.47%> (-19.12%)	⬇️

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[yeounoh]

It's failing to build due to missing google package. I will add a notebook to demonstrate the usage, before review.

[BeibinLi]

@yeounoh In your test case, you can move the "import" line into the existing "try...catch..." clause.

[ekzhu]

@yeounoh Is this PR ready to be reviewed?

[rysweet]

Hi @yeounoh - we've rebased and updated this for you. there are a couple of conflicts still. If you think this is ready for review please update to resolve the conflicts and then we will review.