Add ChatCompletionCache along with AbstractStore for caching completions

[srjoglekar246]

Why are these changes needed?

Add a caching wrapper to ChatCompletionClient, that can use any store with get/set methods for storage.

Related issue number

Closes #4752

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.
• Added unit tests

Copilot reviewed 5 out of 6 changed files in this pull request and generated no comments.

Files not reviewed (1)

• python/packages/autogen-core/src/autogen_core/store/abstract_store_base.py: Evaluated as low risk

[srjoglekar246]

@ekzhu do we need any documentation changes? I suppose API Reference gets updated automatically?

[codecov]

Codecov Report

Attention: Patch coverage is 96.29630% with 3 lines in your changes missing coverage. Please review.

Project coverage is 68.58%. Comparing base (a427b38) to head (652333d).

Files with missing lines	Patch %	Lines
...ages/autogen-core/src/autogen_core/_cache_store.py	66.66%	2 Missing ⚠️
...ges/autogen-core/src/autogen_core/models/_cache.py	98.38%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #4924       +/-   ##
===========================================
+ Coverage   55.71%   68.58%   +12.87%     
===========================================
  Files          68      158       +90     
  Lines        5781    10152     +4371     
===========================================
+ Hits         3221     6963     +3742     
- Misses       2560     3189      +629     

Flag	Coverage Δ
unittests	68.58% <96.29%> (+12.87%)	⬆️

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[ekzhu]

Aside from API documentation. We can discuss the cache in the following places:

https://github.com/microsoft/autogen/blob/main/python/packages/autogen-core/docs/src/user-guide/agentchat-user-guide/tutorial/models.ipynb

https://github.com/microsoft/autogen/blob/main/python/packages/autogen-core/docs/src/user-guide/core-user-guide/framework/model-clients.ipynb

[srjoglekar246]

@ekzhu added docs as you mentioned :-)

[jackgerrits]

I think we should spend some more time to see if the interface can be improved. Especially if this is going into core