chore: move types out of config.rs into config_types.rs (#1054)

`config.rs` is already quite long without these definitions. Since they
have no real dependencies of their own, let's move them to their own
file so `config.rs` can focus on the business logic of loading a config.

codex-rs/core/src/message_history.rs

@@ -28,7 +28,7 @@ use tokio::io::AsyncReadExt;
 use uuid::Uuid;
 
 use crate::config::Config;
-use crate::config::HistoryPersistence;
+use crate::config_types::HistoryPersistence;
 
 #[cfg(unix)]
 use std::os::unix::fs::OpenOptionsExt;