feat(admin, admin-ui, medusa-js, medusa-react, medusa): Support Admin Extensions (#4761)

Co-authored-by: Rares Stefan <948623+StephixOne@users.noreply.github.com>
Co-authored-by: Oli Juhl <59018053+olivermrbl@users.noreply.github.com>

packages/admin-ui/src/client/index.ts

@@ -0,0 +1,16 @@
+export { forbiddenRoutes } from "../../ui/src/constants/forbidden-routes"
+export { injectionZones } from "../../ui/src/constants/injection-zones"
+export type {
+  Extension,
+  ForbiddenRoute,
+  InjectionZone,
+  RouteConfig,
+  RouteExtension,
+  RouteProps,
+  SettingConfig,
+  SettingExtension,
+  SettingProps,
+  WidgetConfig,
+  WidgetExtension,
+  WidgetProps,
+} from "../../ui/src/types/extensions"