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/dashboard/src/lib/validation.ts

@@ -0,0 +1,47 @@
+import i18next from "i18next"
+import { z } from "zod"
+import { castNumber } from "./cast-number"
+
+/**
+ * Validates that an optional value is an integer.
+ */
+export const optionalInt = z
+  .union([z.string(), z.number()])
+  .optional()
+  .refine(
+    (value) => {
+      if (value === "" || value === undefined) {
+        return true
+      }
+
+      return Number.isInteger(castNumber(value))
+    },
+    {
+      message: i18next.t("validation.mustBeInt"),
+    }
+  )
+  .refine(
+    (value) => {
+      if (value === "" || value === undefined) {
+        return true
+      }
+
+      return castNumber(value) >= 0
+    },
+    {
+      message: i18next.t("validation.mustBePositive"),
+    }
+  )
+
+/**
+ * Schema for metadata form.
+ */
+export const metadataFormSchema = z.array(
+  z.object({
+    key: z.string(),
+    value: z.unknown(),
+    isInitial: z.boolean().optional(),
+    isDeleted: z.boolean().optional(),
+    isIgnored: z.boolean().optional(),
+  })
+)