A simple acknowledger that sends a message back to a caller.

It must be stored as:

acknowledger: Broadway.CallerAcknowledger.init({pid, ref}, term)

The first parameter is a tuple with the pid to receive the messages and a unique identifier (usually a reference). The second parameter, which is per message, is ignored.

It sends a message in the format:

{:ack, ref, successful_messages, failed_messages}

If Broadway.Message.configure_ack/2 is called on a message that uses this acknowledger, then the following message is sent:

{:configure, ref, options}