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/admin-shared/package.json

@@ -0,0 +1,27 @@
+{
+  "name": "@medusajs/admin-shared",
+  "description": "Shared code for Medusa admin packages.",
+  "version": "0.0.1",
+  "author": "Kasper Kristensen <kasper@medusajs.com>",
+  "types": "dist/index.d.ts",
+  "main": "dist/index.js",
+  "module": "dist/index.mjs",
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/medusajs/medusa",
+    "directory": "packages/admin/admin-shared"
+  },
+  "files": [
+    "dist",
+    "package.json"
+  ],
+  "scripts": {
+    "build": "tsup"
+  },
+  "devDependencies": {
+    "@types/react": "^18.3.2",
+    "tsup": "^8.0.2",
+    "typescript": "^5.3.3"
+  },
+  "packageManager": "yarn@3.2.1"
+}