feat(utils): DML generates foreign key indexes by default (#7855)

**what:**

- adds default indexes on foreign keys when belongs to relationship is present

RESOLVES CORE-2404

packages/core/utils/src/dal/mikro-orm/decorators/foreign-key.ts

@@ -0,0 +1,10 @@
+import { MetadataStorage } from "@mikro-orm/core"
+
+export function ForeignKey() {
+  return function (target, propertyName) {
+    const meta = MetadataStorage.getMetadataFromDecorator(target.constructor)
+    const prop = meta.properties[propertyName] || {}
+    prop["isForeignKey"] = true
+    meta.properties[prop.name] = prop
+  }
+}