[Auth] Add keyring support for Codex CLI (#5591)

Follow-up PR to #5569. Add Keyring Support for Auth Storage in Codex CLI
as well as a hybrid mode (default to persisting in keychain but fall
back to file when unavailable.)

It also refactors out the keyringstore implementation from rmcp-client
[here](https://github.com/openai/codex/blob/main/codex-rs/rmcp-client/src/oauth.rs)
to a new keyring-store crate.

There will be a follow-up that picks the right credential mode depending
on the config, instead of hardcoding `AuthCredentialsStoreMode::File`.

codex-rs/keyring-store/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+edition = "2024"
+name = "codex-keyring-store"
+version = { workspace = true }
+
+[lints]
+workspace = true
+
+[dependencies]
+keyring = { workspace = true }
+tracing = { workspace = true }