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/features/BUILD.bazel

@@ -0,0 +1,16 @@
+load("//:defs.bzl", "codex_rust_crate")
+
+codex_rust_crate(
+    name = "features",
+    crate_name = "codex_features",
+    compile_data = glob(
+        include = ["**"],
+        exclude = [
+            "BUILD.bazel",
+            "Cargo.toml",
+        ],
+        allow_empty = True,
+    ) + [
+        "//codex-rs:node-version.txt",
+    ],
+)