feat(dashboard): rework create variant flow (#9132)

**What**
- new Create variant flow which allows for pricing and inventory creation as well

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https://github.com/user-attachments/assets/75ddcf5a-0f73-40ca-b474-2189c5e2e297

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CLOSES CC-345

packages/core/types/src/http/product/admin/payloads.ts

@@ -30,6 +30,11 @@ export interface AdminCreateProductVariantPrice {
   rules?: { region_id: string } | null
 }
 
+export interface AdminCreateProductVariantInventoryKit {
+  inventory_item_id: string
+  required_quantity?: number
+}
+
 export interface AdminCreateProductVariant {
   title: string
   sku?: string
@@ -49,6 +54,7 @@ export interface AdminCreateProductVariant {
   material?: string
   metadata?: Record<string, unknown>
   prices: AdminCreateProductVariantPrice[]
+  inventory_items?: AdminCreateProductVariantInventoryKit[]
   options?: Record<string, string>
 }