feat: Add basic CRUD functionality to store module (#6510)

Adds the core model and basic CRUD around it.

Note: The store model is not complete, but I prefer doing smaller PRs so it's easier to do a proper review. Adding currencies will be a follow-up PR.

packages/store/integration-tests/__fixtures__/index.ts

@@ -0,0 +1,10 @@
+import { StoreTypes } from "@medusajs/types"
+
+export const createStoreFixture: StoreTypes.CreateStoreDTO = {
+  name: "Test store",
+  default_sales_channel_id: "test-sales-channel",
+  default_region_id: "test-region",
+  metadata: {
+    test: "test",
+  },
+}