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/core/tests/suite/model_switching.rs

@@ -1,8 +1,8 @@
 use anyhow::Result;
 use codex_core::CodexAuth;
 use codex_core::config::types::Personality;
-use codex_core::features::Feature;
 use codex_core::models_manager::manager::RefreshStrategy;
+use codex_features::Feature;
 use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::config_types::ServiceTier;
 use codex_protocol::openai_models::ConfigShellToolType;