Add ChatCompletionCache along with AbstractStore for caching completions #3521

Job	Run time
lint	28s
mypy (./packages/autogen-core)	16s
format	24s
mypy (./packages/autogen-magentic-one)	24s
pyright (./packages/autogen-core)	16s
mypy (./packages/agbench)	17s
pyright (./packages/autogen-magentic-one)	16s
mypy (./packages/autogen-ext)	31s
mypy (./packages/autogen-agentchat)	19s
pyright (./packages/agbench)	16s
mypy (./packages/magentic-one-cli)	16s
pyright (./packages/autogen-ext)	18s
pyright (./packages/autogen-agentchat)	16s
pyright (./packages/magentic-one-cli)	18s
check-proto-changes-python	44s
codecov (./packages/autogen-core)	22s
codecov (./packages/autogen-magentic-one)	17s
docs (./packages/autogen-core)	18s
codecov (./packages/autogen-ext)	25s
docs-example-check (./packages/autogen-core)	18s
codecov (./packages/autogen-agentchat)	16s
markdown-code-lint	17s
samples-code-check	25s
test (./packages/autogen-core)	24s
test (./packages/autogen-magentic-one)	24s
test (./packages/autogen-ext)	17s
test (./packages/autogen-agentchat)	17s
	9m 19s

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