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/src/lib.rs

@@ -29,6 +29,7 @@ pub mod exec;
 mod exec_command;
 pub mod exec_env;
 pub mod executor;
+pub mod features;
 mod flags;
 pub mod git_info;
 pub mod landlock;