feat: move create inventory to @medusajs/workflows (#5301)

**Why**
- We have some workflow-like flows in @medusajs/medusa. These should be moved over to the workflows package.
- Inventory Items <> Variant currently assume a 1-1 mapping. There should be support for a many-to-many mapping.

**What**
- PR introduces a feature flag for supporting many-to-many mappings for inventory and variants.
- Deletes legacy transaction handler in @medusajs/medusa.
- Adjusts existing createInventoryItems handler to remove dependency on variant data.

**Unkowns**
~~1. Couldn't find an existing test for the CreateProduct workflow. It should be tested that this still works as expected.~~
2. Have removed transaction managers as we should move to handling consistency through orchestration tooling. Are we ready for that?

packages/utils/src/feature-flags/index.ts

@@ -6,4 +6,4 @@ export * from "./sales-channels"
 export * from "./tax-inclusive-pricing"
 export * from "./utils"
 export * from "./workflows"
-
+export * from "./many-to-many-inventory"

packages/utils/src/feature-flags/many-to-many-inventory.ts

@@ -0,0 +1,9 @@
+import { FeatureFlagTypes } from "@medusajs/types"
+
+export const ManyToManyInventoryFeatureFlag: FeatureFlagTypes.FlagSettings = {
+  key: "many_to_many_inventory",
+  default_val: false,
+  env_key: "MEDUSA_FF_MANY_TO_MANY_INVENTORY",
+  description:
+    "Enable capability to have many to many relationship between inventory items and variants",
+}