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/Cargo.toml

@@ -13,6 +13,7 @@ base64 = { workspace = true }
 chrono = { workspace = true }
 codex-app-server-protocol = { workspace = true }
 codex-core = { workspace = true }
+codex-features = { workspace = true }
 codex-protocol = { workspace = true }
 codex-utils-cargo-bin = { workspace = true }
 serde = { workspace = true }