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/.gitignore

@@ -1,4 +1,5 @@
 /dist
 /build
 .vercel
-/ui/preview
+/ui/preview
+/ui/src/extensions