Switch to the format being drafted in open-telemetry/semantic-conventions#2179, hopefully close to merging.

There will be a new param in InstrumentationSettings to switch between the current format using events/all_messages_events and the new one using gen_ai.input/output.messages (the internal shape is also quite different). The param event_mode won't be allowed with the new format. The new format will be the default in v1.