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>

ui/src/api/index.ts

@@ -12,4 +12,5 @@ export { costsApi } from "./costs";
 export { activityApi } from "./activity";
 export { dashboardApi } from "./dashboard";
 export { heartbeatsApi } from "./heartbeats";
+export { instanceSettingsApi } from "./instanceSettings";
 export { sidebarBadgesApi } from "./sidebarBadges";