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/prompt_caching.rs

@@ -5,6 +5,7 @@ use codex_core::ConversationManager;
 use codex_core::ModelProviderInfo;
 use codex_core::built_in_model_providers;
 use codex_core::config::OPENAI_DEFAULT_MODEL;
+use codex_core::features::Feature;
 use codex_core::model_family::find_family_for_model;
 use codex_core::protocol::AskForApproval;
 use codex_core::protocol::EventMsg;
@@ -99,10 +100,10 @@ async fn codex_mini_latest_tools() {
     config.cwd = cwd.path().to_path_buf();
     config.model_provider = model_provider;
     config.user_instructions = Some("be consistent and helpful".to_string());
+    config.features.disable(Feature::ApplyPatchFreeform);
 
     let conversation_manager =
         ConversationManager::with_auth(CodexAuth::from_api_key("Test API Key"));
-    config.include_apply_patch_tool = false;
     config.model = "codex-mini-latest".to_string();
     config.model_family = find_family_for_model("codex-mini-latest").unwrap();
 
@@ -185,7 +186,7 @@ async fn prompt_tools_are_consistent_across_requests() {
     config.cwd = cwd.path().to_path_buf();
     config.model_provider = model_provider;
     config.user_instructions = Some("be consistent and helpful".to_string());
-    config.include_plan_tool = true;
+    config.features.enable(Feature::PlanTool);
 
     let conversation_manager =
         ConversationManager::with_auth(CodexAuth::from_api_key("Test API Key"));