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/generated/graphql.tsx

@@ -778,6 +778,7 @@ export type Index = {
   __typename?: 'Index';
   createdAt: Scalars['DateTime'];
   id: Scalars['UUID'];
+  indexFieldMetadataList: Array<IndexField>;
   indexFieldMetadatas: IndexIndexFieldMetadatasConnection;
   indexType: IndexType;
   indexWhereClause?: Maybe<Scalars['String']>;