Add UserProxyAgent in AgentChat API (#4255) #1539

Job	Run time
docs (./packages/autogen-core)	48s
format	19s
lint	18s
mypy (./packages/autogen-core)	1m 23s
pyright (./packages/autogen-core)	38s
mypy (./packages/autogen-magentic-one)	50s
test (./packages/autogen-core)	51s
test (./packages/autogen-magentic-one)	59s
mypy (./packages/agbench)	27s
pyright (./packages/autogen-magentic-one)	36s
test (./packages/autogen-ext)	1m 13s
mypy (./packages/autogen-ext)	53s
pyright (./packages/agbench)	36s
test (./packages/autogen-agentchat)	35s
mypy (./packages/autogen-agentchat)	45s
check-proto-changes-python	41s
pyright (./packages/autogen-ext)	35s
pyright (./packages/autogen-agentchat)	38s
	13m 5s

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