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/medusa/package.json

@@ -46,7 +46,7 @@
   "dependencies": {
     "@inquirer/checkbox": "^2.3.11",
     "@inquirer/input": "^2.2.9",
-    "@medusajs/admin-sdk": "0.0.1",
+    "@medusajs/admin-bundler": "0.0.1",
     "@medusajs/core-flows": "^0.0.9",
     "@medusajs/framework": "^0.0.1",
     "@medusajs/link-modules": "^0.2.11",