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-bundler/src/lib/develop.ts

@@ -0,0 +1,33 @@
+import express from "express"
+import type { InlineConfig } from "vite"
+
+import { BundlerOptions } from "../types"
+import { getViteConfig } from "./config"
+
+const router = express.Router()
+
+export async function develop(options: BundlerOptions) {
+  const vite = await import("vite")
+
+  try {
+    const viteConfig = await getViteConfig(options)
+
+    const developConfig: InlineConfig = {
+      mode: "development",
+      logLevel: "warn",
+    }
+
+    const server = await vite.createServer(
+      vite.mergeConfig(viteConfig, developConfig)
+    )
+
+    router.use(server.middlewares)
+  } catch (error) {
+    console.error(error)
+    throw new Error(
+      "Failed to start admin development server. See error above."
+    )
+  }
+
+  return router
+}