Ideally, it should be capable of outputting any type which is returned by the endpoint processing lambda.

Both HttpServerSourceStage and HttpClientSourceStage support a payload_to_df_fn method, however the output is a DataFrame.

If we wanted to do this I think there are two options:

1. Break API compatibility and change this to a request_to_msg_fn method
2. Add a second optional method to convert both the payload and the DataFrame to a message.

However currently for HttpServerSourceStage specifying payload_to_df_fn forces the Python impl of the stage (although the server itself is still implemented in C++).

I'm thinking instead doing something similar to what the DeserializeStage does and do:

1. Add a message_type & task_type constructor arguments
2. Add a request_to_task_payload_fn constructor argument. When None a default C++ impl will be used. When not-none the Python source stage impl will be used.