Cannot configure custom conversation visualizer with conversation stats

[malhotra5]

Currently, in order to pass a custom visualizer for the agent-sdk we need

1. define a custom visualizer class with an on_event method
2. pass the visualizer in the conversation callbacks arg (callbacks=[custom_vizualizer.on_event])

However, the conversation stats only exist once the conversation is created. So we can't plumb conversation_stats into the visualizer.

Clients may want the ability to configure their own visualizers. For example, we don't want to emit the system prompt in OpenHands CLI

[malhotra5]

Solved by #1025