feat(core-flows,medusa): add endpoint to add/remove fulfillment providers to location (#8299)

what:

- add endpoint to add/remove fulfillment providers to location
- specs cleanup from previous PR

RESOLVES CC-259

integration-tests/modules/__tests__/shipping-options/store/shipping-options.spec.ts

@@ -134,16 +134,14 @@ medusaIntegrationTestRunner({
               fulfillment_set_id: fulfillmentSet.id,
             },
           },
-          {
-            [Modules.STOCK_LOCATION]: {
-              stock_location_id: stockLocation.id,
-            },
-            [Modules.FULFILLMENT]: {
-              fulfillment_provider_id: "manual_test-provider",
-            },
-          },
         ])
 
+        await api.post(
+          `/admin/stock-locations/${stockLocation.id}/fulfillment-providers`,
+          { add: ["manual_test-provider"] },
+          adminHeaders
+        )
+
         shippingOption = (
           await api.post(
             `/admin/shipping-options`,