Add ItemStarted/ItemCompleted events for UserInputItem (#5306)

Adds a new ItemStarted event and delivers UserMessage as the first item
type (more to come).


Renames `InputItem` to `UserInput` considering we're using the `Item`
suffix for actual items.

codex-rs/protocol/src/lib.rs

@@ -2,9 +2,11 @@ mod conversation_id;
 pub use conversation_id::ConversationId;
 pub mod config_types;
 pub mod custom_prompts;
+pub mod items;
 pub mod message_history;
 pub mod models;
 pub mod num_format;
 pub mod parse_command;
 pub mod plan_tool;
 pub mod protocol;
+pub mod user_input;