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/Cargo.toml

@@ -29,7 +29,7 @@ ts-rs = { workspace = true, features = [
     "serde-json-impl",
     "no-serde-warnings",
 ] }
-uuid = { workspace = true, features = ["serde", "v7"] }
+uuid = { workspace = true, features = ["serde", "v7", "v4"] }
 
 [dev-dependencies]
 anyhow = { workspace = true }