Introduce rollout items (#3380)

This PR introduces Rollout items. This enable us to rollout eventmsgs
and session meta.

This is mostly #3214 with rebase on main

codex-rs/core/src/rollout/mod.rs

@@ -10,7 +10,6 @@ pub mod recorder;
 pub use recorder::RolloutRecorder;
 pub use recorder::RolloutRecorderParams;
 pub use recorder::SessionMeta;
-pub use recorder::SessionStateSnapshot;
 
 #[cfg(test)]
 pub mod tests;