Split features into codex-features crate (#15253)

- Split the feature system into a new `codex-features` crate.
- Cut `codex-core` and workspace consumers over to the new config and
warning APIs.

Co-authored-by: Ahmed Ibrahim <219906144+aibrahim-oai@users.noreply.github.com>
Co-authored-by: Codex <noreply@openai.com>

codex-rs/app-server/tests/common/config.rs

@@ -1,5 +1,5 @@
-use codex_core::features::FEATURES;
-use codex_core::features::Feature;
+use codex_features::FEATURES;
+use codex_features::Feature;
 use std::collections::BTreeMap;
 use std::path::Path;