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__/cart/store/cart.workflows.spec.ts

@@ -1499,16 +1499,11 @@ medusaIntegrationTestRunner({
             },
           ])
 
-          await remoteLink.create([
-            {
-              [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
+          )
 
           const shippingOption = await fulfillmentModule.createShippingOptions({
             name: "Test shipping option",

integration-tests/modules/__tests__/cart/store/carts.spec.ts

@@ -1941,16 +1941,11 @@ medusaIntegrationTestRunner({
             },
           ])
 
-          await remoteLink.create([
-            {
-              [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(