core: add extra metadata field to Thread struct (#29675)

# Summary

Adds a field Thread.extras that can be used to hold arbitrary metadata
specific to a given thread.

codex-rs/analytics/src/client_tests.rs

@@ -277,6 +277,7 @@ fn sample_thread_archive_request() -> ClientRequest {
 fn sample_thread(thread_id: &str) -> Thread {
     Thread {
         id: thread_id.to_string(),
+        extra: None,
         session_id: format!("session-{thread_id}"),
         forked_from_id: None,
         parent_thread_id: None,