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/Cargo.lock

@@ -1887,6 +1887,7 @@ dependencies = [
  "codex-login",
  "codex-network-proxy",
  "codex-otel",
+ "codex-plugin",
  "codex-protocol",
  "codex-rmcp-client",
  "codex-rollout",
@@ -2394,6 +2395,17 @@ dependencies = [
  "zip",
 ]
 
+[[package]]
+name = "codex-plugin"
+version = "0.0.0"
+dependencies = [
+ "codex-utils-absolute-path",
+ "codex-utils-plugins",
+ "serde",
+ "serde_json",
+ "thiserror 2.0.18",
+]
+
 [[package]]
 name = "codex-process-hardening"
 version = "0.0.0"