functional api: Add ability to request previous output

[eyurtsev]

1. The inputs into foo do not affect any state behavior
2. previous always reflects the previous return value from the function
3. Anything can be returned and that will be the new state for the function on the next iteration
4. This API is not meant to support reducers in the inputs/state

  from langgraph.func import entrypoint

  states = []

  # In this version reducers do not work
  @entrypoint(checkpointer=MemorySaver())
  def foo(inputs, *, previous: Any) -> Any:
      states.append(previous)
      return {"previous": previous, "current": inputs}

  config = {"configurable": {"thread_id": "1"}}

  foo.invoke({"a": "1"}, config)
  foo.invoke({"a": "2"}, config)
  foo.invoke({"a": "3"}, config)
  assert states == [
      None,
      {"current": {"a": "1"}, "previous": None},
      {"current": {"a": "2"}, "previous": {"current": {"a": "1"}, "previous": None}},
  ]

[nfcampos]

Please don't remove generator support

[eyurtsev]

How do you want to deal with the return value In the generator case? Why should we support it?

[nfcampos]

why is this changing?