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/ui/src/main.tsx

@@ -1,9 +1,22 @@
 import ReactDOM from "react-dom/client"
 import "./assets/styles/global.css"
 import MedusaApp from "./medusa-app"
+import { ExtensionsEntry } from "./types/extensions"
 
 async function run() {
-  const app = new MedusaApp()
+  let exts: ExtensionsEntry[] = []
+
+  try {
+    const { default: extensions } = (await import(
+      // @ts-ignore - this file is generated at build time
+      "./extensions/_main-entry.ts"
+    )) as { default: ExtensionsEntry[] }
+    exts = extensions
+  } catch (_) {
+    // noop - no extensions
+  }
+
+  const app = new MedusaApp({ entries: exts })
 
   ReactDOM.createRoot(document.getElementById("root") as HTMLElement).render(
     app.render()