Move auth code into login crate (#15150)

- Move the auth implementation and token data into codex-login.
- Keep codex-core re-exporting that surface from codex-login for
existing callers.

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/login/src/auth/mod.rs

@@ -0,0 +1,10 @@
+pub mod default_client;
+pub mod error;
+mod storage;
+mod util;
+
+mod manager;
+
+pub use error::RefreshTokenFailedError;
+pub use error::RefreshTokenFailedReason;
+pub use manager::*;