feat(core-flows,modules-sdk,types,medusa,link-modules): adds variant <> inventory item link endpoints (#7576)

what:

- adds variant inventory link management endpoints:
```
Link inventory item to variant
POST /products/:id/variants/:vid/inventory-items

Update variant's inventory item link
POST /products/:id/variants/:vid/inventory-items/:iid

Unlink variant's inventory item
DELETE /products/:id/variants/:vid/inventory-items/:iid
```

- a batch endpoint that does the above 3 across variants
```
POST /products/:id/variants/inventory-items
```

packages/modules/link-modules/src/services/dynamic-service-class.ts

@@ -1,11 +1,23 @@
 import { Constructor, ILinkModule, ModuleJoinerConfig } from "@medusajs/types"
+import { isDefined } from "@medusajs/utils"
 import { LinkModuleService } from "@services"
 
 export function getModuleService(
   joinerConfig: ModuleJoinerConfig
 ): Constructor<ILinkModule> {
   const joinerConfig_ = JSON.parse(JSON.stringify(joinerConfig))
+  const databaseConfig = joinerConfig_.databaseConfig
+
   delete joinerConfig_.databaseConfig
+
+  // If extraDataFields is not defined, pick the fields to populate and validate from the
+  // database config if any fields are provided.
+  if (!isDefined(joinerConfig_.extraDataFields)) {
+    joinerConfig_.extraDataFields = Object.keys(
+      databaseConfig.extraFields || {}
+    )
+  }
+
   return class LinkService extends LinkModuleService<unknown> {
     override __joinerConfig(): ModuleJoinerConfig {
       return joinerConfig_ as ModuleJoinerConfig