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/config_profile.rs

@@ -20,6 +20,18 @@ pub struct ConfigProfile {
     pub model_verbosity: Option<Verbosity>,
     pub chatgpt_base_url: Option<String>,
     pub experimental_instructions_file: Option<PathBuf>,
+    pub include_plan_tool: Option<bool>,
+    pub include_apply_patch_tool: Option<bool>,
+    pub include_view_image_tool: Option<bool>,
+    pub experimental_use_unified_exec_tool: Option<bool>,
+    pub experimental_use_exec_command_tool: Option<bool>,
+    pub experimental_use_rmcp_client: Option<bool>,
+    pub experimental_use_freeform_apply_patch: Option<bool>,
+    pub tools_web_search: Option<bool>,
+    pub tools_view_image: Option<bool>,
+    /// Optional feature toggles scoped to this profile.
+    #[serde(default)]
+    pub features: Option<crate::features::FeaturesToml>,
 }
 
 impl From<ConfigProfile> for codex_app_server_protocol::Profile {