Make `register_factory` a user facing API #2886

Job	Run time
format	51s
lint	59s
mypy (./packages/autogen-core)	2m 23s
pyright (./packages/autogen-core)	59s
mypy (./packages/autogen-magentic-one)	1m 20s
pyright (./packages/autogen-magentic-one)	57s
mypy (./packages/agbench)	54s
pyright (./packages/agbench)	55s
mypy (./packages/autogen-ext)	1m 53s
pyright (./packages/autogen-ext)	1m 2s
mypy (./packages/autogen-agentchat)	1m 9s
pyright (./packages/autogen-agentchat)	1m 4s
check-proto-changes-python	1m 8s
docs (./packages/autogen-core)	1m 19s
docs-example-check (./packages/autogen-core)	2m 24s
test (./packages/autogen-core)	57s
test (./packages/autogen-magentic-one)	1m 22s
test (./packages/autogen-ext)	3m 2s
test (./packages/autogen-agentchat)	1m 8s
	25m 46s

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