Add metadata field to basemessage #5103

Job	Run time
lint	1m 12s
format	52s
mypy (./packages/autogen-core)	3m 38s
mypy (./packages/agbench)	1m 0s
mypy (./packages/autogen-ext)	3m 7s
check-proto-changes-python	1m 9s
mypy (./packages/autogen-agentchat)	1m 25s
docs (./packages/autogen-core)	1m 43s
mypy (./packages/magentic-one-cli)	1m 30s
docs-example-check (./packages/autogen-core)	3m 37s
pyright (./packages/autogen-core)	1m 2s
samples-code-check	56s
pyright (./packages/agbench)	1m 2s
pyright (./packages/autogen-ext)	1m 21s
test (./packages/autogen-core)	1m 10s
test-grpc	1m 29s
pyright (./packages/autogen-agentchat)	1m 3s
test (./packages/autogen-ext)	4m 10s
pyright (./packages/magentic-one-cli)	1m 6s
test (./packages/autogen-agentchat)	1m 5s
markdown-code-lint	3m 9s
codecov (./packages/autogen-core)	8s
codecov (./packages/autogen-ext)	8s
codecov (./packages/autogen-agentchat)	8s
codecov (autogen-ext-grpc)	9s
	37m 19s

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