Extract codex-utils-plugins crate (#15746)

## Summary
- extract shared plugin path and manifest helpers into
codex-utils-plugins
- update codex-core to consume the utility crate

## Testing
- CI

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

codex-rs/Cargo.toml

@@ -69,6 +69,7 @@ members = [
     "utils/oss",
     "utils/output-truncation",
     "utils/path-utils",
+    "utils/plugins",
     "utils/fuzzy-match",
     "utils/stream-parser",
     "utils/template",
@@ -163,6 +164,7 @@ codex-utils-json-to-toml = { path = "utils/json-to-toml" }
 codex-utils-oss = { path = "utils/oss" }
 codex-utils-output-truncation = { path = "utils/output-truncation" }
 codex-utils-path = { path = "utils/path-utils" }
+codex-utils-plugins = { path = "utils/plugins" }
 codex-utils-pty = { path = "utils/pty" }
 codex-utils-readiness = { path = "utils/readiness" }
 codex-utils-rustls-provider = { path = "utils/rustls-provider" }