fix IndexFieldMetadata availability in IndexMetadata/ObjectMetadata in front (#12886)

Context : 
- IndexFieldMetadata was no longer available on 'objects' gql query
([since this PR](https://github.com/twentyhq/twenty/pull/12785)). Then,
unicity checks on import do not work anymore.

Fix : 
- Add a dataloader logic in indexFieldMetadata
- Add extra check in unicity hook on import

packages/twenty-front/src/modules/object-metadata/utils/mapPaginatedObjectMetadataItemsToObjectMetadataItems.ts

@@ -1,3 +1,4 @@
+import { IndexFieldMetadataItem } from '@/object-metadata/types/IndexFieldMetadataItem';
 import { IndexMetadataItem } from '@/object-metadata/types/IndexMetadataItem';
 import { objectMetadataItemSchema } from '@/object-metadata/validation-schemas/objectMetadataItemSchema';
 import { ObjectMetadataItemsQuery } from '~/generated-metadata/graphql';
@@ -26,7 +27,12 @@ export const mapPaginatedObjectMetadataItemsToObjectMetadataItems = ({
           (index) =>
             ({
               ...index,
-              indexFieldMetadatas: [],
+              indexFieldMetadatas: index.indexFieldMetadataList.map(
+                (indexFieldMetadata) =>
+                  ({
+                    ...indexFieldMetadata,
+                  }) satisfies IndexFieldMetadataItem,
+              ),
             }) satisfies IndexMetadataItem,
         ),
       } satisfies ObjectMetadataItem;