feat(dashboard): ability to locate new admin route under existing route (#10587)

This PR add ability to locate new admin route under existing route in sidebar.

For example, new route Brands
![image](https://github.com/user-attachments/assets/1b297fb0-296c-4e94-a4cb-f84f4c676c53)
![image](https://github.com/user-attachments/assets/80336909-1c0a-49c9-b8e1-3b1137ae2e48)

https://github.com/user-attachments/assets/b46b1813-e92e-4b67-84a1-84660023ac7c

packages/admin/admin-sdk/src/config/types.ts

@@ -4,6 +4,7 @@ import type {
   CustomFieldModelContainerMap,
   CustomFieldModelFormTabsMap,
   InjectionZone,
+  NestedRoutePosition,
 } from "@medusajs/admin-shared"
 import type { ComponentType } from "react"
 import { ZodFirstPartySchemaTypes } from "zod"
@@ -24,6 +25,11 @@ export interface RouteConfig {
    * An optional icon to display in the sidebar together with the label. If no label is provided, the icon will be ignored.
    */
   icon?: ComponentType
+
+  /**
+   * The nested route to display under existing route in the sidebar.
+   */
+  nested?: NestedRoutePosition
 }
 
 export type CustomFormField<