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/src/extensions/routes/constants.ts

@@ -0,0 +1 @@
+export const ROUTE_IMPORTS = ["routes/pages", "routes/links"] as const

packages/admin/admin-shared/src/extensions/routes/index.ts

@@ -0,0 +1,2 @@
+export * from "./constants"
+export * from "./types"

packages/admin/admin-shared/src/extensions/routes/types.ts

@@ -0,0 +1,3 @@
+import { ROUTE_IMPORTS } from "./constants"
+
+export type RouteImport = (typeof ROUTE_IMPORTS)[number]