chore(dependencies): make medusa-interfaces a peerDependency

packages/medusa-plugin-discount-generator/package.json

@@ -31,6 +31,9 @@
     "watch": "babel -w src --out-dir . --ignore **/__tests__",
     "test": "jest"
   },
+  "peerDependencies": {
+    "medusa-interfaces": "1.x"
+  },
   "dependencies": {
     "body-parser": "^1.19.0",
     "express": "^4.17.1",