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/src/commands/build.ts

@@ -154,7 +154,7 @@ export default async function ({ directory }: BuildArgs) {
   if (!adminOptions.disable) {
     try {
       const { build: buildProductionBuild } = await import(
-        "@medusajs/admin-sdk"
+        "@medusajs/admin-bundler"
       )
 
       await buildProductionBuild(adminOptions)

packages/medusa/src/loaders/admin.ts

@@ -52,7 +52,7 @@ export default async function adminLoader({
 }
 
 async function initDevelopmentServer(app: Express, options: IntializedOptions) {
-  const { develop } = await import("@medusajs/admin-sdk")
+  const { develop } = await import("@medusajs/admin-bundler")
 
   const adminMiddleware = await develop(options)
   app.use(options.path, adminMiddleware)
@@ -60,7 +60,7 @@ async function initDevelopmentServer(app: Express, options: IntializedOptions) {
 }
 
 async function serveProductionBuild(app: Express, options: IntializedOptions) {
-  const { serve } = await import("@medusajs/admin-sdk")
+  const { serve } = await import("@medusajs/admin-bundler")
 
   const adminRoute = await serve(options)