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/graphql/queries.ts

@@ -33,6 +33,13 @@ export const FIND_MANY_OBJECT_METADATA_ITEMS = gql`
             indexWhereClause
             indexType
             isUnique
+            indexFieldMetadataList {
+              id
+              fieldMetadataId
+              createdAt
+              updatedAt
+              order
+            }
           }
           fieldsList {
             id