Extract codex-plugin crate (#15747)

## Summary
- extract plugin identifiers and load-outcome types into codex-plugin
- update codex-core to consume the new plugin crate

## Testing
- CI

---------

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

codex-rs/plugin/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+edition.workspace = true
+license.workspace = true
+name = "codex-plugin"
+version.workspace = true
+
+[lib]
+doctest = false
+name = "codex_plugin"
+path = "src/lib.rs"
+
+[lints]
+workspace = true
+
+[dependencies]
+codex-utils-absolute-path = { workspace = true }
+codex-utils-plugins = { workspace = true }
+thiserror = { workspace = true }