Add instance experimental setting for isolated workspaces

Introduce a singleton instance_settings store and experimental settings API, add the Experimental instance settings page, and gate execution workspace behavior behind the new enableIsolatedWorkspaces flag.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

packages/shared/src/validators/index.ts

@@ -1,3 +1,10 @@
+export {
+  instanceExperimentalSettingsSchema,
+  patchInstanceExperimentalSettingsSchema,
+  type InstanceExperimentalSettings,
+  type PatchInstanceExperimentalSettings,
+} from "./instance.js";
+
 export {
   upsertBudgetPolicySchema,
   resolveBudgetIncidentSchema,