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/features/legacy.rs

@@ -0,0 +1,158 @@
+use super::Feature;
+use super::Features;
+use tracing::info;
+
+#[derive(Clone, Copy)]
+struct Alias {
+    legacy_key: &'static str,
+    feature: Feature,
+}
+
+const ALIASES: &[Alias] = &[
+    Alias {
+        legacy_key: "experimental_use_unified_exec_tool",
+        feature: Feature::UnifiedExec,
+    },
+    Alias {
+        legacy_key: "experimental_use_exec_command_tool",
+        feature: Feature::StreamableShell,
+    },
+    Alias {
+        legacy_key: "experimental_use_rmcp_client",
+        feature: Feature::RmcpClient,
+    },
+    Alias {
+        legacy_key: "experimental_use_freeform_apply_patch",
+        feature: Feature::ApplyPatchFreeform,
+    },
+    Alias {
+        legacy_key: "include_apply_patch_tool",
+        feature: Feature::ApplyPatchFreeform,
+    },
+    Alias {
+        legacy_key: "include_plan_tool",
+        feature: Feature::PlanTool,
+    },
+    Alias {
+        legacy_key: "include_view_image_tool",
+        feature: Feature::ViewImageTool,
+    },
+    Alias {
+        legacy_key: "web_search",
+        feature: Feature::WebSearchRequest,
+    },
+];
+
+pub(crate) fn feature_for_key(key: &str) -> Option<Feature> {
+    ALIASES
+        .iter()
+        .find(|alias| alias.legacy_key == key)
+        .map(|alias| {
+            log_alias(alias.legacy_key, alias.feature);
+            alias.feature
+        })
+}
+
+#[derive(Debug, Default)]
+pub struct LegacyFeatureToggles {
+    pub include_plan_tool: Option<bool>,
+    pub include_apply_patch_tool: Option<bool>,
+    pub include_view_image_tool: Option<bool>,
+    pub experimental_use_freeform_apply_patch: Option<bool>,
+    pub experimental_use_exec_command_tool: Option<bool>,
+    pub experimental_use_unified_exec_tool: Option<bool>,
+    pub experimental_use_rmcp_client: Option<bool>,
+    pub tools_web_search: Option<bool>,
+    pub tools_view_image: Option<bool>,
+}
+
+impl LegacyFeatureToggles {
+    pub fn apply(self, features: &mut Features) {
+        set_if_some(
+            features,
+            Feature::PlanTool,
+            self.include_plan_tool,
+            "include_plan_tool",
+        );
+        set_if_some(
+            features,
+            Feature::ApplyPatchFreeform,
+            self.include_apply_patch_tool,
+            "include_apply_patch_tool",
+        );
+        set_if_some(
+            features,
+            Feature::ApplyPatchFreeform,
+            self.experimental_use_freeform_apply_patch,
+            "experimental_use_freeform_apply_patch",
+        );
+        set_if_some(
+            features,
+            Feature::StreamableShell,
+            self.experimental_use_exec_command_tool,
+            "experimental_use_exec_command_tool",
+        );
+        set_if_some(
+            features,
+            Feature::UnifiedExec,
+            self.experimental_use_unified_exec_tool,
+            "experimental_use_unified_exec_tool",
+        );
+        set_if_some(
+            features,
+            Feature::RmcpClient,
+            self.experimental_use_rmcp_client,
+            "experimental_use_rmcp_client",
+        );
+        set_if_some(
+            features,
+            Feature::WebSearchRequest,
+            self.tools_web_search,
+            "tools.web_search",
+        );
+        set_if_some(
+            features,
+            Feature::ViewImageTool,
+            self.include_view_image_tool,
+            "include_view_image_tool",
+        );
+        set_if_some(
+            features,
+            Feature::ViewImageTool,
+            self.tools_view_image,
+            "tools.view_image",
+        );
+    }
+}
+
+fn set_if_some(
+    features: &mut Features,
+    feature: Feature,
+    maybe_value: Option<bool>,
+    alias_key: &'static str,
+) {
+    if let Some(enabled) = maybe_value {
+        set_feature(features, feature, enabled);
+        log_alias(alias_key, feature);
+    }
+}
+
+fn set_feature(features: &mut Features, feature: Feature, enabled: bool) {
+    if enabled {
+        features.enable(feature);
+    } else {
+        features.disable(feature);
+    }
+}
+
+fn log_alias(alias: &str, feature: Feature) {
+    let canonical = feature.key();
+    if alias == canonical {
+        return;
+    }
+    info!(
+        %alias,
+        canonical,
+        "legacy feature toggle detected; prefer `[features].{canonical}`"
+    );
+}