@Nonnull
    default ConversationChannelAgent<T, R> newAgent(ConversationChannelListener<T, R> listener) {
        ConversationChannelAgent<T, R> agent = newAgent();
        agent.addListener(listener);
        return agent;
    }
Creates a new {@link ConversationChannelAgent} for this channel, and
immediately adds the given {@link ConversationChannelListener} to it.

@param listener the listener to register
@return the resultant agent
helper/src/main/java/me/lucko/helper/messaging/conversation/ConversationChannel.java