Base class for parsing agent output into agent action/finish.
Parses a message into agent action/finish.
Is meant to be used with OpenAI models, as it relies on the specific function_call parameter from OpenAI to convey what tools to use.
If a function_call parameter is passed, then that is used to get the tool and tool input.
If one is not passed, then the AIMessage is assumed to be the final output.
