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/core/types/src/admin/extensions.ts

@@ -9,13 +9,13 @@
  * @example
  * ```tsx
  * import type { DetailWidgetProps, AdminProduct } from "@medusajs/types"
- * import { defineConfig } from "@medusajs/admin-shared"
+ * import { defineWidgetConfig } from "@medusajs/admin-sdk"
  *
  * const ProductWidget = ({ data }: DetailWidgetProps<AdminProduct>) => {
  *  return <div>{data.title}</div>
  * }
  *
- * export const config = defineConfig({
+ * export const config = defineWidgetConfig({
  *  zone: "product.details.after",
  * })
  *