feat: feature flag (#4948)

Add proper feature flag instead of having custom flags for everything.
This is just for experimental/wip part of the code
It can be used through CLI:
```bash
codex --enable unified_exec --disable view_image_tool
```

Or in the `config.toml`
```toml
# Global toggles applied to every profile unless overridden.
[features]
apply_patch_freeform = true
view_image_tool = false
```

Follow-up:
In a following PR, the goal is to have a default have `bundles` of
features that we can associate to a model

codex-rs/core/tests/suite/tools.rs

@@ -2,6 +2,7 @@
 #![allow(clippy::unwrap_used, clippy::expect_used)]
 
 use anyhow::Result;
+use codex_core::features::Feature;
 use codex_core::model_family::find_family_for_model;
 use codex_core::protocol::AskForApproval;
 use codex_core::protocol::EventMsg;
@@ -293,7 +294,11 @@ async fn collect_tools(use_unified_exec: bool) -> Result<Vec<String>> {
     let mock = mount_sse_sequence(&server, responses).await;
 
     let mut builder = test_codex().with_config(move |config| {
-        config.use_experimental_unified_exec_tool = use_unified_exec;
+        if use_unified_exec {
+            config.features.enable(Feature::UnifiedExec);
+        } else {
+            config.features.disable(Feature::UnifiedExec);
+        }
     });
     let test = builder.build(&server).await?;