feat(admin-sdk,admin-bundler,admin-shared,medusa): Restructure admin packages (#8988)

**What**
- Renames /admin-next -> /admin
- Renames @medusajs/admin-sdk -> @medusajs/admin-bundler
- Creates a new package called @medusajs/admin-sdk that will hold all tooling relevant to creating admin extensions. This is currently `defineRouteConfig` and `defineWidgetConfig`, but will eventually also export methods for adding custom fields, register translation, etc. 
  - cc: @shahednasser we should update the examples in the docs so these functions are imported from `@medusajs/admin-sdk`. People will also need to install the package in their project, as it's no longer a transient dependency.
  - cc: @olivermrbl we might want to publish a changelog when this is merged, as it is a breaking change, and will require people to import the `defineXConfig` from the new package instead of `@medusajs/admin-shared`.
- Updates CODEOWNERS so /admin packages does not require a review from the UI team.

packages/admin/dashboard/src/lib/query-client.ts

@@ -0,0 +1,13 @@
+import { QueryClient } from "@tanstack/react-query"
+
+export const MEDUSA_BACKEND_URL = __BACKEND_URL__ ?? "http://localhost:9000"
+
+export const queryClient = new QueryClient({
+  defaultOptions: {
+    queries: {
+      refetchOnWindowFocus: false,
+      staleTime: 90000,
+      retry: 1,
+    },
+  },
+})